Engine rattling noise at idle - Mercedes-Benz Forum
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#1 (permalink) Old 02-09-2009, 06:04 PM
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Engine rattling noise at idle

My 98 E320 70k miles is making a rattling sound from the engine bay at idle once the engine has warmed up. It can only be heard outside the car. I took off the plastic cover and can't seem to pinpoint where it is coming from. Any ideas?
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#2 (permalink) Old 02-09-2009, 07:09 PM
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Could it be coming from the exhaust cats? Are you certain that the noise isn't coming from your catalytic converter(s)? Lean under the car near the front doors and see if the noise is stronger under the car. Your miles are about right for a cat failure. If that's it... $600 each.

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#3 (permalink) Old 02-09-2009, 08:06 PM
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Hey cg. Please click the user cp link and complete your BW profile re vehicle and location as that is often useful information.

Here, for example, if you are in the US -- but NOT CA -- and it is your cats, there are universal cats that can be welded in quite affordably.

However, you need to properly diagnose this. The only noise I can think of by your terse description is also cats. But you have FOUR cats, two on each side, and if you can't isolate it you'll have to take it in. A mechanic's stethoscope will help isolate such noises.

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#4 (permalink) Old 02-09-2009, 09:07 PM
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Thanks for the info. Are the 4 cats located underneath the engine? It sounds like it's coming more from the top of the engine and something is loose and rattling. I will check to see if the noise is louder under the car tomorrow.
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#5 (permalink) Old 02-09-2009, 09:49 PM
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Since my car is under 80k miles, would MB replace the cats under warranty?
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#6 (permalink) Old 02-09-2009, 10:11 PM
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Nope, it's out of the 8 year limit.



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#7 (permalink) Old 02-09-2009, 10:55 PM
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Originally Posted by cgrizzly888 View Post
Thanks for the info. Are the 4 cats located underneath the engine? It sounds like it's coming more from the top of the engine and something is loose and rattling. I will check to see if the noise is louder under the car tomorrow.
The primary/front cats are visible from the engine compartment, just ahead of the position 1 O2 sensor and after the header. The secondary/rear cats are laterally close to the front of the transmission underneath the car after the position 2 sensor. There have been problems with both, I think in the earliest V6 years it was more the primary cats, then the secondary.

Don't make this harder than it needs to be. Get a mechanic's steth and you'll easily isolate it. If you have a Harbor Freight nearby they're like $3. You'll need it anyway to isolate other noises like WP bearings, tensioner bearings, idler bearings, etc.

Take care and enjoy the ride,
Greg

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#8 (permalink) Old 02-09-2009, 11:48 PM
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Is it a loud

rattle or a light drum like ticking?
On the top side of the engine in front of the brake booster is a object labeled MOT,that is your purge valve.It will tap away like a little drummer.Normal.
If under the car,get it on a lift and check for the sound coming from the cats.You are in NJ so eastern cats has universal unit that are about $50 each and can be spliced in by any good custom exhaust shop.
Keep us posted ,when you locate the sound.And as always be careful working around moving parts while trying to locate a sound with the car running.


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#9 (permalink) Old 02-10-2009, 07:51 AM
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I didn't get a steth yet, however, I noticed my belt tensioner has excessive movent. It moves side to side while the engine is moving. I wonder if this is causing the rattle noise.
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#10 (permalink) Old 02-10-2009, 11:12 AM
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I didn't get a steth yet, however, I noticed my belt tensioner has excessive movent. It moves side to side while the engine is moving. I wonder if this is causing the rattle noise.
That would indeed do it. If it has free play (check it with the engine OFF, just grab it and move it up/down, in/out top out/bottom in and vice-versa) then stop driving the car and replace it. If it has enough free play to make noise and be felt/seen it is due to let go at any time and you'll be stranded. If you don't want to wait for shipping just get the part at your dealer, obviously it will cost more but you'll be on the road sooner. You should be able to just buy the pulley with the bearing instead of the whole tensioner assembly, that's lots cheaper and usually the part that fails anyway.

Take care and enjoy the ride,
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