Knock knock sound on takeoff/slowdown, non-engine issue
 

Hi all,

On my '85 300D, I've noticed a sound coming from within the floorboard that sounds like someone knocking on a door. It's inconsistent, and usually only happens at lower speeds when I start accelerating, then sometimes when I slow down. I've done some research and some have said it's suspension related. Any thoughts here?

If it is under acceleration, there is a good chance it could be from the driveshaft wobbling. Maybe your center bearing is out to lunch. If the frequency of knocking increases with speed that is a good clue. Otherwise yes, probably suspension related, but you need to isolate the noise more carefully, perhaps drive by an observer or something like that.

Good luck.

Chris W.
former '95 E300D, 467K

Could even be a cv joint. Yet they usually are more a click click noise in the early stages of failure.

Also the side the noise is on can be identified by someone in the back seat going to one open window than the other usually. That's if you do not sense where it is.
Doing this eliminates them as a possibility as well. Other than that a basic inspection should indicate the fault.

Guide rod

Another thing you could do is get under the car and inspect the rubber items associated with the drive line. You'll want something to pry on them with a bit. Visually they might appear good but when you flex the rubber you can see cracks and what not.

Mine made a thumping sound under load when going up a hill or sometimes under harder acceleration. I looked and everything seemed visually ok, but leveraged the trans mount with a prybar and could see bad cracks.