126 300 SE Mystery Noise

[NKowi]

Just got a 91 300SE and have noticed a sort of rumbling noise when under medium or greater acceleration. It's particularly noticeable during left turns. I've not done a complete check of the exhaust system to see if something's a bit loose (I should be so lucky). If it's not something as simple as that, I'm hopeful that someone here has experience with the same sort of thing and can offer some advice as to what I should look for.
Thanks!

[Dave Horvath]

I will have to watch your posts. I to have a noise like that on my 89 300SE.
Mine seams to be louder when the car is cold and I pull out of the driveway.

[NKowi]

I wondered if it was my imagination - worse when cold. Maybe we can both get some advice here.

[wbain5280]

It may be the driveshaft center bearing support.

Here is the manual, section 41.

http://mb.braingears.com/126_DISC1/program/chassis_300SE.htm

[86560SEL]

Could be the exhaust system. My mothers 1997 Oldsmobile has started doing this recently, especially before the engine is fully warmed up and at highway speeds. We have found out it was a brace on the exhaust had broken. It was still original, so I guess it was time it broke. Not bad for a later model American car.

[Rob Pruijt]

The things I would check first are; 1 engine mounts, 2 wheel bearings.

[ILUVMILS]

It sounds like the engine mounts are sagging. The exhaust system can come into contact with the underbody cross-member. With the car on a lift it's easy to see where it's rubbing. This is a pretty common problem on 126's with the M103.

[NKowi]

Thanks to each of you. I'll get it in the air this weekend and try to identify the problem.

[fahrgewehr2]

Look at the exhaust where it would come into contact with the crossmember and you may see a mark where it rubs. Change trans mount in addition to motor mounts.

[dave geibel]

I have an 89 300se with the same problem. Only started 200 miles ago, and seems to be getting worse. My noise begins around 35 mph and increases with more speed. Noise continues even when coasting in neutral and goes away around 25 mph. I had the wheels checked for balance. Tires checked for bad belts, and bearings are new. After many searches, the drive shaft area keeps coming up, and that's next. After looking at the manual, it doesn't look like a diy project. I'll be anxious to see any more helpful posts.

[S-Class Guru]

Been there, done that on my '91 300SE.
In my opinion, you guys probably have different problems. There are 4 possibilities for alll:

1. If you get a rumbling/bumping under acceleration, especially left turns, it's probably the exhaust hitting the cross member just below the right rear of the engine, as the guys said. The right motor mount collapses, and the combo of engine torque and centrifugal force when turning left make the right side drop. Sitting still, it can clear by 1/4 inch, and still bump during the conditions above.
A quick fix is to place 3 washers under the 3 right-side cross-member bolts, and see if the problem goes away. Then replace the motor mounts.

2. If the rumble is pretty constant, even without the power on, then it is likely a driveshaft. Try the front flex disc first - easy to replace without removing anything else, if you're a pretty good contortionist. The rear
doesn't seem to fail too often.
if both discs seem good (replace the front anyway, just to be sure)
then it's probably the center bearing.
This can be done by the DIY'r, but it's a pain.
- Drop the entire exhaust from the manifold back.
- Drop the shield.
- Drop both ends and the center of the shaft.
- check the center bearing, or just replace it - it's pretty cheap.
- mark the mating points of the front and rear halves carefully before
pulling the drive shaft apart.
- try to ignore the little detent/flat-spot in the U-joint, unless it's severe. This requires big bucks and a good speciality shop.

3. Might be a rear axle. You can switch the rear axles, and put the cV joints in a different load position. this worked for me on a 70 mph buzz under load.

After having said all that, I still have a slight vibration between 15-17 MPH under brisk acceleration. I know it's driveline, cause it does it in either low or 2nd gear at the same car speed, and it's about 3.5 times wheel speed.
So, I assume the evil u-joint with the detent is finally mis-behaving.
But so far, I've resisted the urge to go thru a u-joint replacement.

4. Wheel bearing of course. Should be easy to feel by turning it.
I have one bearing that has zero end play, and the other has had some play since new. but no problems in 175k miles on either. So end play is not an
indicator of bad problems I don't think. Listen/feel for roughness when turning the wheel.

Good luck.
DG

[NKowi]

Removed the three bolts from passengers' side cross-member, inserted about 4mm of washers between member and frame, re-installed bolts, went on test drive. No more noise. Will replace pass-side motor mount as soon I can get a new one. Thanks for the help!!!

[Dave Horvath]

I finally got around to changing my motor mounts. Sitting on the bench there was almost 1'' differance between the new and old one. The passenger side was the worst. Not to bad of a job to do. About 3 hours with no lift.