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Discussion Starter #1
1991 300SEL.

It's a grinding or rumbling noise from under the hood. It only happens when the car is in a drive gear, and it will come and go. It only makes the noise at a stop, and it won't always make the noise at a stop. I might wait a minute at a stop light and the it will start but it only ever lasts at most thirty seconds. The problem is that it's loud as all hell with the hood open, and you can barely hear it with the radio on while driving.

Anybody had similar experiences?
 

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I have the same problem! Tranny was serviced and motor mounts were replaced about 7 months ago (around 140k miles) on my '91 300SE.

420SE how do you know it has to do with the a/c compressor?

I always notice the grinding when I'm at a stop in Drive. If i switch to Neutral, the grinding stops.. that's how I ruled out the brakes. Other than that I'm still stuck! :crybaby2:
 

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My 300SE does the same thing. It's not the A/C compressor, at least on mine, as I barely ever use the A/C except when it's very hot in the summer. i like heat. I almost always use the economy setting.

I assumed it was the motor mounts. But you said that was done to yours, and it's still doing it... Huh.

Mine gives a clunk when starting, which I'm sure is the motor mounts, and the noise only occurs with the car in gear, especially at stop lights, and under load, such as when accellerating.

It's very annoying, and if it's NOT motor mounts, I'm dying to see if anyone figures out just what it is.

Aaron
 

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Aaron,
Check the bottom of the fan shroud, if you see scrapes there,or the edges of the fan are brighter, then it's your mounts. Your "...only in Drive and under acceleration.." kinda sounds like motor mounts to me.
 

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First thing that comes to my mind is the air pump bearing. I had to do mine twice in about 6 years. Or any of the other accessory component bearings (alternator, A/C comp, water pump or the aforementioned air pump bearing). However it may be the exhaust pipe banging on the trans crossmember, as that is known to happen also. That is an easy one to diagnose as all you have to do is look at it to see if there is clearance between the pipe and the crossmember.
 

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I think the shroud sounds about right. If the shroud was removed for any reason while the motor mounts were on the way out, it would have been positioned too low. When you eventually had the mounts replaced the motor would sit higher and lead to the fan rubbing on the inner top portion of the shroud. The fan clutching and de-clutching could explain why it is intermittent.

GT
 

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I've done some more digging that leads me to believe it's the motor mounts. I'll have to buy the OE. ones.

http://www.benzworld.org/forums/w126-s-se-sec-sel-sd/1279244-ok-just-vent-all-300-series.html

That is to an old thread, and it states that the aftermarket ones are trash, and that could be gr33nspan's trouble, if they were replaced with aftermarket.

Apparently the noise is caused by the exhaust rubbing on some suspension/steering components.

I have thought it was the motor mounts for a while, and I just planned on replacing them in the spring on principle. I guess they don't last very long- Looking at the service history of the car, which now has 155,000 miles on it, they've been replaced 4 times on both sides in the last 60,000 miles. Perhaps they were replaced by an indy mechanic with cheapos every time. It's certainly well worth the investment of the dealer parts to avoid these problems.

Aaron
 

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Discussion Starter #13
My fan's not making the noise. The motor mounts and tranny mount were replaced three months ago, so they're good. There's clearance between my exhaust and cross member, but the bearings in one of the pulleys might be going. I've been told to check my belt tensioner but it's a few hours to get it replaced...
 
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