Knocking/Rapping Noise
 

Please help. I am trying to diagnose a sharp knocking or rapping noise coming from the left rear of my '88 420SEL. It happens on acceleration and goes away when I ease off of the gas.

After poking around under the car, I thought the culprit was the
sway bar links, the left side felt loose and clicked when manipulated. So I replaced both sides. Noise went away for 2 days, then came back. Any suggestions?

Thanks,
Mike

knocking at rear on acceleration
 

could it be exhaust pipe hitting? Check all the hangers to make sure they are OK.

Exhaust hangers look good. The bottom-out bumpers are intact.
I tried to recreate the noise by shaking the exhaust, nothing.

In other posts that the symptoms sound close, it has been
suggested that it might be the axle halfs.
Can anyone tell me how to check them without using "replace & see" technique?

Thanks again!

Mike

hi

you didn't mention how the rapping sounded. does it change with the rpm. does it do it only while moving or does it sound off at a stand still? does it sound off on acceleration or deceleration. have you ruled out the engine compartment?

i am replacing a camshaft this week on my 420sel. i would describe that as a sharp rapping also. mine lunched a rocker arm wore down so bad i have to replace the camshaft as well.

with a bit more description we (the forum) may just figure this one out.

Hi Craig,

This is definately from the rear.
Only on accelleration in drive. More rapid and louder earlier under accel. and diminishes as it gets up to speed and I back off the throttle.

Does not happen when parked and revving engine.
I was able to reproduce it slightly by standing on the brake and revving the engine while in drive. I had a friend outside the car confirm that it was from the rear drivers side.


Thanks,
Mike

Mike I am having the same problem on my 300E. I suspect the axle joints especially the outboard one on the drivers side to be on its last leg. I only have the noise under acceleration and in right turns only. Back off and i t goes away. Its getting worse so its just a matter of time before the truth is known. If its not the axle then it must be the wheel bearing. But I hope it is something less expensive.

Dave,

I think you are probably right about the axle joints.
Any idea how to test them? Can I possibly inject some grease into the joint to see if it stops the noise (temporarily)?

Thanks,
Mike

I have never been into a MB type. I understand that they use an oil rather than grease like is typical of CV joints. I am in uncharted waters here. Hopefully someone else with more experience or knowledge will chime in. I doubt that injecting grease will cure them. I suspect that they are worn out. Not sure how to test them either.

Dave,

I'm thinking that injecting grease into the joint would simply confirm the diagnosis, not cure it.
How difficult a job is it to replace the shafts? Any tips, anyone?

Thanks,

Mike

Mike, explain how grease would confirm the diagonsis. I have the CD rom which details the proceedure. Looks like one might need special tools to get the job done. Certainly will if the wheel bearing is to be replaced.

Changing them on the 300D was by far one of the easiest diyer type jobs I have done. I'm sure ya'll model is just as easy.

Sorry for the delay in getting back to you Dave. It's tough when your employer keeps you busy at work, and the wife keeps you busy at home!

I am working on the (probably wrong) assumption that the joint is making noise do to a lack of lubrication.

My thinking is that by injecting grease into the joint, the new grease would quiet it down temporarily (since it would no longer be sealed).

Regards,
Mike