1998 E300 Tapping/Cracking/Tock Tock Sound From Rear as Accelerating
 

My 1998 E300 (102,250 miles) has developed an interesting knocking sound from the rear. The sound usually happens when I am accelerating from a stop and is most audible with my window down. In fact, if I am next to another car, the tapping (like a noisy tock tock tock which increases with speed) echoes very loudly. This is definitely an "under load" sound but I don't know what part in the rear can possibly make such a sporadic knock under acceleration. Could it be a flex disk or driveshaft issue? It's more of an annoyance than anything else, but I would still love some opinions.

The CV joints are worn out, or it could possibly be a bad wheel bearing.

I'll go along with that
 

Better check those CV joints in the rear axles. Sounds like you have one that is about ready to come apart on you.

I would first inspect the CV boots for tears. Then give it a good look around the drivetrain while on a rack. Then I'd see if the sound could be detected while running in gear on the rack. Possible wheel bearing?

Let us know what you find out.

Thanks for all the replies! I noticed that the CV issue is more prevalent on W123 cars than W210--only threads on 210 relates to front CVs on 4Matic cars. I went out and looked at the CV boots on the diesel and all the rubber components seem to be intact. What should I look for?

That tap tap tap increasing with speed from a stop is only intermittent and usually occurs in 3rd gear (if that means anything??). Thanks!

Update
 

No new developments from the CV side...but the tapping noise seems to be coming from the LEFT back tire...any ideas?