I have a 1991 350SD with 250,000 miles. It's always been well maintained and is a great car. Recently, I have noticed a barely perceptible rumble from the rear end when starting off. It lasts a few seconds then goes away and there is no noise at any other time. The only culprits I can think of would be rear wheel bearings or flex disks. Any insights before I begin my explorations?

You don't specify if the "rumble" is related to engine RPM or vehicle speed.

If vehicle speed, you have wheel bearings (usually this shows up at higher speed), flex disk (again, this usually doesn't go away), suspension components, and differential.

If the engine RPM has anything to do with it, you could have exhaust system mounts, motor mounts, or even transmission mount, all of which can transmit noise to the chassis. If you have a hole forming in part of your exhaust, it would definitely sound like a "rumble", especially when you get on it from a stop since the rest of the car is pretty quiet at that point.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

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1985 524TD 167K (TotalDumpster™)

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1991 560SEL
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1986 500SEL Euro (Rusted to nothing at 47K!)

I replaced the engine mounts, shocks, axles and exhaust hangers within the last 5,000 miles. The exhaust system is patent: No holes. I replaced the differential fluid within the past year. Engine speed doesn't seem to have any effect on it: It happens when the car begins to move from a dead stop. I don't hear the noise when cruising at any speed. It almost sounds like something is rattling around in the trunk. It "rumbles" for about 3 seconds and then goes away. If I had to swear on it, I'd say I can feel a vibration as well. So very faint however.Definitely sounds like its coming from the trunk. My impression is that something is under more stress at the time of accelerating from a dead stop than at any other time and this is what's failing.
I'm by no means an expert mechanic but I have kept this baby together on my own for a long, long time.
Thanks for your input and thoughts.

Check the drive shaft center support bearing and rubber mounting.

__________________
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Give consideration to the condition of the rear diff mount; it is attached to the trunk floor.
When that mount is tits-up, starting torque can cause momentary metal-to-metal contact.

Frank, Mike and Diseasel300,
I'm very appreciative of your help. I'll get under the old girl this weekend and see if I can figure it out. I'll report back. Thanks again.
Carlos

Well I've been under the car and tried to see if anything visible is failing and nothing looks out of sorts. Since any Mercedes mechanic in Dallas charges more than a doctor saving someone's life, I'm going to replace all the rubber on the frame floor bearing (about $400 in parts) and see if that's it. Not the most scientific method but anyone who has ever looked at this car and given me an estimate to do anything (and I mean ANYTHING) comes in around $1500 minimum. If the new rubber on the frame floor bearing doesn't fix it, I'll have to move on to plan "B", whatever that might be.

OK. It's been a bit warm in Dallas and I don't have an air conditioned garage so it has taken me awhile to brave the elements and work on this problem.
I went the easy route first and replaced the differential mount and associated rubber but, alas, the rumble is still with me.
I'm leaning towards Mike D.'s thoughts at this point (drive shaft center support bearing and rubber mount). I may wait until we get a string of 80's before tackling this so it might be October but I will report back.

I was going to go with Mike D as well. Starting off sounds like center support. Jack up the rear and put the car in neutral and get under and rotate the drive shaft. You may need to remove the exhaust heat shield if there is one there to access the shaft.

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1990 190E 3.0L

Given the years and miles, flex discs might be a good idea anyway.

Absolutely agree! Have you checked it?

I'm on the second one after only 84K miles on my W201. First was on warranty and the second was about 1000 miles ago. The first occurance was obvious as a growling sound was coming up from the center consul, which indicated a spalled bearing.

The second was weird. There was a squeaking noise coming from the same area, but only at about 15 MPH.

The center driveshaft bearing is mounted in a rubber cushioned steel pillow block. I had to remove the catalyst heat shield to see it, but it was easy to remove the three screws and work it out. Then I grabbed the shaft near the bearing and could freely move it. Closer visual inspection reveled that portions of the rubber were missing. Heat from the catalyst was probably a factor, despite the heat shield.

This wasn't a job I wanted to tackle with the car up on jackstands, but I have a good indy near me in Hermosa Beach, A & R German Motors. I drove the car down first thing in the morning and Ralph went right to work. Labor was two hours, and the total job was about $330 including a new bearing and pillowblock. He let me watch and inspect the driveshaft when it was out. The front "flex disk", which is really a torsional vibration damper (replaced at 60K) was in good shape as were the front and rear flex couplings. The u-joint had a very slight notch, but given I only drive the car about 500 miles per year Ralph said it was okay for now. Had the u-joint needed replacement he would have had to send the driveshaft to an outside shop for a full rebuild.

While doing my inspection for the noise, I highlighted the matchmarks on two driveshaft pieces with a yellow tire crayon to be sure it got assembled with proper indexing. The dealer who replaced the flexdisk didn't do that, which resulted in a slight vibration. They got to redo the job!

The center driveshaft bearing/pillowblock isn't discussed much here, but I've had to do two replacements in a little over 80K miles. It should be inspected every time the car is up in the air for service - just grab and wiggle the driveshaft as close to the center bearing as possible to check if it flops around.

Duke