Knocking Noise From Under The Car

[Tom Evans]

I have a pair of 126 300SDs (1981 and 1982). The 1981 has developed a knocking noise that is coming from the left rear seat area. The noise occurs at 1200 RPM in all gears. The noise comes and goes, but appears to be the worst when it is cold. I took out the rear seat bottom and rode around with a friend. He said he could "feel" the noise coming directly from under the left rear seat area. You get the noise whether you are accelerating or de-accelerating.

I have thoroughly checked the area for any traces of metal rubbing or things bumping together. I have renewed all of the exhaust hanger mounts, both on the muffler and resonators. I checked the subframe mount on that side of the body. Since the flex disks had almost 300K on them, I replaced them as well although that shouldn't be the problem since the problem is engine speed related, not road speed related.

Could it be the left resonator have some internal problem? Neither resonator shows any external evidence of damage or dents.

Any ideas?

Many thanks.

[Air&Road]

Noises and vibrations are among the most difficult things to troubleshoot from afar. That said it could easily be something internal to a muffler.

Hope this helps.

[SD Blue]

If this noise occurs only while in motion, I would suspect sway bar links.


1981 Mercedes-Benz 300SD Sedan - Suspension, Shocks & Springs - Page 3

[ROLLGUY]

Sounds to me like an axle noise. Does the sound change when turning a corner?

[Brian Carlton]

Originally Posted by Tom Evans

The noise occurs at 1200 RPM in all gears.

If the noise occurs at 1200 rpm, will it be noticed in neutral at that engine speed?

Is the noise related to the motion of the vehicle or the engine speed?


The problem cannot be the axles or the swaybar links if the noise ONLY occurs at 1200 rpm as these items will cause vibration relative to road speed and/or road irregularities.

[Tom Evans]

Let me address your suggestions/questions:

The noise is not dependent on the direction you are turning - it occurs equally in all directions.

Since it is resonant at 1200 RPM, the car is basically just starting to roll in 1st gear, so I don't think it is drive train related.

Several times, I thought I heard it while standing still in neutral at 1200 RPM.

It is definitely more noticeable when the engine is under load.

The motor mounts are fairly new Lemforder mounts (fewer than 5K miles) and there is lots of clearance between the motor support arms and the mount lip.

Since the last time, I put everything back together (drive shaft and exhaust system), there are times when the noise is completely gone and then it will be back the next time I hit 1200 RPM.

The scary thing is that my other car which is newer, but has more miles, is starting to have his noise, but only rarely.

[4x4_Welder]

That really sounds like an exhaust issue. That car has the giant dual mufflers that wing out under the back seats. Make sure the heat shields are all in good shape and there isn't an exhaust leak.