Wierd Oscillating Noise at Highway Speeds - Mercedes-Benz Forum
 
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#1 (permalink) Old 09-18-2010, 07:57 PM
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Wierd Oscillating Noise at Highway Speeds

Hey,

I have a 83 300D Turbo, and I one of the last things that I did to my car was I jacked up the rear end, put jack stands underneath the subframe bushings, and replaced my rear calipers, rotors, pads, and while I was at it I replaced both axles too.

Since I did that job, when I get it up to between 75-80 mph on the speedo (I think it is actually about 5 mph slower than that), I get a loud waah-waah-waah vibration in the car (seems to be more on drivers side maybe). Sounds a little like if you drive over the rumble strips on the side of the highway, but the noise oscillates in amplitude at around maybe 60Hz or so. Also, the noise is much worse/louder when it is hotter out, on cold mornings recently it is MUCH better...not gone, but much better.

I had done the front brakes before the rear and a friend suggested that the bearings needed to be replaced and were causing the noise. Replaced both front bearings with no change.

I thought it might be the center bearing/support on the drive shaft. I replaced that and it was in pretty rough shape but no change with the noise.

I noticed that the driver side motor mount was trashed and thought that it was obvious that this must be the culprit. Replaced both motor mounts (I had replaced the one at the back of the transmission when I changed the center bearing), but no change with the noise.

So now I am thinking that it has to be something directly related to the rear of the car since it started after I worked on the rear. The rear does seem to sit lower now that I remember it before, and sometimes going over speed bumps, etc. I will bottom out the back end which never happened before.

So does anyone have any ideas what could be causing the noise?

I have been toying with the idea that it could be the subframe bushings and when I put the jackstands under them before it did something to them, but could that cause the noise? I looked at them and they don't look horrible, but they are 27 years old now.

Could it be that my shocks in the rear are gone? That could be why my car sinks lower, but could it be causing the noise?

Thanks for any help and suggestions!
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#2 (permalink) Old 09-18-2010, 08:04 PM
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When you figure it out, let me know. I have not owned a W123 that doesn't do it, and my 107 does also. At first I thought it was a diesel thing, but the 107 is a V8 gas. My W126 doesn't do it.

If your car is squatting sitting still, it probably needs springs, shocks won't fix that. If it's squatting while taking off, that's another story. Do the old 'bounce' test to verify the condition of the shocks.

Shocks won't get rid of your noise, however, I replaced mine and the noise persists.

The only thing I can think of is that it's some slight imbalance of the CV shafts.

Another thought I had- could the rear wheels be spinning at a slightly different rate?

-Aaron
ASE Master tech, specializing in electronics and driveability.
Certified Diesel Master tech.
EPA 609 certified

1973 450SL - 191K - Summer Toy
1978 300D - 267K
1978 300CD - drove to death
1981 300D - drove to death
1983 300CD -170K
1989 300SE 105K miles..
1981 Coupe DeVille.. just because.
... and many more past and present...



Because Safety is not a luxury. Mercedes-Benz. Engineered like no other car in the world.
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#3 (permalink) Old 09-18-2010, 08:22 PM
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Wow, that's not what I wanted to hear!

I NEVER had this problem until that rear end work. How could I check to see if the rear wheels are spinning at different rates? I'm not sure I can think of a good reason why replacing the axles would cause this.

I have actually had a very similar noise on another car when I had a wheel that had an issue (out of balance or had some imperfection), I have tried getting the wheels balanced...nothing. And I have tried putting the spare in place of the front drivers side wheel....nothing. I will try moving that spare around to the other three positions, but I doubt that will fix the issue.
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#4 (permalink) Old 09-18-2010, 09:14 PM
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Sorry for no good suggestion, but I have a rumble in my '84 300DT (370k mi.) starting at 55 mph. I had the rear axles replaced, mostly because the right's inner boot had broke open some time ago, and I put a Speedi-Boot over it that I had left over from another vehicle, so it was just a temp fix. Anyway, the rumble was still there after the axle replacements, but no difference in the rumble -- starts at the same speed, same frequency. My center support has a little crack in the rubber, but not sure how little 'til I replace it. The one on my '73 220D had a rumble after reaching a certain speed, and that was the support that time. Weird that I replaced that at about 100K mi and 8-yrs, and this one in the 300DT has 370K mi and is 26-yrs old. I was/am not the orig owner of either car, but I'm sure neither one had the support replaced before I bought them. The 220D had more winter exposure (1st 4 yrs in Germany, then 5 in Nebraska (where I replaced the support), then TX and GA 'til 240K mi. The 300DT, 1st 11 yrs and 103K mi in FL, then GA (with me).

I'll get the support replaced n another few weeks -- doing the oil cooler lines this weekend. If the rumble goes away, then we'll know that a new support can be a cure, though it didn't fix yours.

Dave H.

'73 220D White "White Knight" ('77-'06, 240K mi; floorboards rusted; gone to MB Heaven)
'84 300D Turbo Maroon "Raspberry" ('95-present, 375K mi; daughter named him)
'95 Merc Villager "Blueberry2" ('00-present, 252K mi-2nd engine-tip of crankshaft broke at 170K)
'97 S320 LWB Black "Ebony" (2010-present, 217K; bought at 204K mi)
'07 Dark-Green GMC Yukon Denali "Jade" (Added Apr 2011 - 104K)

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#5 (permalink) Old 09-19-2010, 05:43 AM
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Rear Shock absorbers out of alignment?

Quote:
Originally Posted by kurtept View Post
Hey,

I have a 83 300D Turbo, and I one of the last things that I did to my car was I jacked up the rear end, put jack stands underneath the subframe bushings, and replaced my rear calipers, rotors, pads, and while I was at it I replaced both axles too.

Since I did that job, when I get it up to between 75-80 mph on the speedo (I think it is actually about 5 mph slower than that), I get a loud waah-waah-waah vibration in the car (seems to be more on drivers side maybe). Sounds a little like if you drive over the rumble strips on the side of the highway, but the noise oscillates in amplitude at around maybe 60Hz or so. Also, the noise is much worse/louder when it is hotter out, on cold mornings recently it is MUCH better...not gone, but much better.

I had done the front brakes before the rear and a friend suggested that the bearings needed to be replaced and were causing the noise. Replaced both front bearings with no change.

I thought it might be the center bearing/support on the drive shaft. I replaced that and it was in pretty rough shape but no change with the noise.

I noticed that the driver side motor mount was trashed and thought that it was obvious that this must be the culprit. Replaced both motor mounts (I had replaced the one at the back of the transmission when I changed the center bearing), but no change with the noise.

So now I am thinking that it has to be something directly related to the rear of the car since it started after I worked on the rear. The rear does seem to sit lower now that I remember it before, and sometimes going over speed bumps, etc. I will bottom out the back end which never happened before.

So does anyone have any ideas what could be causing the noise?

I have been toying with the idea that it could be the subframe bushings and when I put the jackstands under them before it did something to them, but could that cause the noise? I looked at them and they don't look horrible, but they are 27 years old now.

Could it be that my shocks in the rear are gone? That could be why my car sinks lower, but could it be causing the noise?

Thanks for any help and suggestions!

Is it possible that you may have mis-aligned your rear shock absorbers while working on the rear of the car?

I am by no means an expert at all, but i just happened to be glancing at my service manual and came across a section where they warn about making sure that rear shock absorbers are properly aligned. This may not be it at all, but just trying to help..
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#6 (permalink) Old 09-19-2010, 06:05 AM
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Alignment could be out or uneven tread wear? Sometimes I attribute a wahwahwahwahwah noise to the tire treads, sometimes from uneven wear, sometimes they just get louder as they wear.

Wheel bearings can produce a grinding sometimes, or a squeaking that disappears when the brakes are applied, or sometimes a noise that comes and goes or quickly varies in intensity. It can also stop depending on the direction you turn. Have you replaced the rear wheel bearings? This would probably match the noise you're describing, but can also be more of a vibrational grinding sound. Like grit and pinballs in a metal bowl being spun.

Does the noise change if you turn the wheel slightly while driving? Get up to speed and then slowly turn the wheel left and then back right. If that changes the noise you may consider looking at your ball joints and steering components. You can check the ball joints by taking pressure off them by supporting the lower control arm. Then use a 2x4 or the like under the tire and pull up on your end to move the tire. If the ball joints show any play, then they are due to be replaced.

Bad bushings can make a car sit lower. Mainly you'll be looking for torn rubber. This would probably produce noise at speed too.

The W126 used to sound like a freight train. I think it was the hard rubber "Runway" brand tires. Never was able to test the hypothesis however.

-1983 300DT: 259,000+
-1986 300E: 143,000+

Last edited by aubs; 09-19-2010 at 06:13 AM.
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#7 (permalink) Old 09-19-2010, 12:53 PM
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Quote:
Originally Posted by aubs View Post
Alignment could be out or uneven tread wear? Sometimes I attribute a wahwahwahwahwah noise to the tire treads, sometimes from uneven wear, sometimes they just get louder as they wear.

Wheel bearings can produce a grinding sometimes, or a squeaking that disappears when the brakes are applied, or sometimes a noise that comes and goes or quickly varies in intensity. It can also stop depending on the direction you turn. Have you replaced the rear wheel bearings? This would probably match the noise you're describing, but can also be more of a vibrational grinding sound. Like grit and pinballs in a metal bowl being spun.

Does the noise change if you turn the wheel slightly while driving? Get up to speed and then slowly turn the wheel left and then back right. If that changes the noise you may consider looking at your ball joints and steering components. You can check the ball joints by taking pressure off them by supporting the lower control arm. Then use a 2x4 or the like under the tire and pull up on your end to move the tire. If the ball joints show any play, then they are due to be replaced.

Bad bushings can make a car sit lower. Mainly you'll be looking for torn rubber. This would probably produce noise at speed too.

The W126 used to sound like a freight train. I think it was the hard rubber "Runway" brand tires. Never was able to test the hypothesis however.
Hmmm, good thoughts. I will look in the manual to see how I would go about aligning the rear suspension.

Still haven't ruled out tire wear since that is just what it sounds like...just haven't been able to find the culprit tire and it seems VERY odd that it was non-existent before the rear end work and SO prevalent after.

I did replace the front bearings but didn't feel like tackling the rears, but the noise is there if I change lanes or take an offramp. As soon as I take my foot off the gas it goes away. Also at the same time this started happening I noticed that when I take an exit at 30 mph turning right, as I am turning there is a thump thump thump noise. This doesn't happen if I do the same turning left. That is why I was thinking it was some support mount or bushing.

Beyond the motor mounts, and the subframe bushings, are there any other mounts or bushings that could be causing this? I am thinking about just replacing the subframe bushings since they are cheap and it can't hurt.

Thanks!
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#8 (permalink) Old 09-19-2010, 01:26 PM
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Checking alignment of rear shock absorbers

I think that you may find section 32-126 of the Chassis and Body Series 123, Vol 1 Service Manual interesting. If you don't have access to it, let me know, and I will try to scan the section for you, or I may be able to get it off of another disk....
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#9 (permalink) Old 09-19-2010, 06:11 PM
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I think that you may find section 32-126 of the Chassis and Body Series 123, Vol 1 Service Manual interesting. If you don't have access to it, let me know, and I will try to scan the section for you, or I may be able to get it off of another disk....
I have it, that is interesting, but I guess I am not sure how replacing the axles would have put this out of alignment. I mean all I did was pull the axle on either end to pop it out, then I slid the new one in. I can't see that I would have done anything to reposition the hub to throw anything else out.

I guess I can't really do the test they suggest anyway, so not sure what I would do if it was out of alignment.
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#10 (permalink) Old 09-19-2010, 08:12 PM
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You can get a four wheel alignment done at most tire shops.

You really should be thinking about bearings before that if you don't see anything wrong with the treads. Maybe you unseated a bearing when you pulled out the axles or knocked something enough to free up a frozen ball bearing. From your description of the noise coming and going, it sounds like a bearing.

A bushing might result in a singular crack noise every once and while.

-1983 300DT: 259,000+
-1986 300E: 143,000+
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