The 300SD we have does that also, although the clunk will sometimes stop without the acc or sometimes, it'll clunk with the acc on. The mechanic said it's something about vacuum, and that the tranny may need to be rebuilt. How many miles do you have on yours?
-Joe

__________________
'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.