Underbody Knocking - or is it?

[Birdman]

It's a light, mechanical repetitive tapping noise apparently coming from the rear offside area usually heard when driving slowly (20mph) over bumpy roads. It produces two or three quick, light taps, then nothing. Even when the boot is emptied of everything including the warning triangle.

Not that I do a lot of miles like this, it's just a case of: what is it?

Btw the tapping stops when I have rear seat passengers on board.

 

[andy_k]

anti roll bar drop link would be my number one suspect.

There's one either side and they cost about £10 each to replace/rule out and can be fitted in 20 minutes (or less)

Andy

 

[nickmann]

Could it be seat-related? either the seat itself, or something underneath it? I had a tapping from the boot and it took ages to find that a penlight battery had found the tiny hole from the bootwell into the spare-wheel well. This won't be what yours is because yours stops when you have weight on the seat, but I reckon it pays to check the easy stuff first.

 

[Alex Argent]

Just a possibility but my C240 was doing the same a couple of years back and dealer found broken rear spring. Apparently it's a common problem on the C Class, not sure about the E though, but would guess the design is similar - at least worth a quick check. The bottom part of the spring sits in a well that is prone to filling with water, which consequently rusts the bottom part of the spring away. It initially sounded like the exhaust was knocking somewhere.
Good luck!

 

[Birdman]

I had this happen rather dramatically with a front spring (story related in http://www.mbclub.co.uk/forums/showthread.php?t=14185 ) and will look into that possibility too.