For what its worth...
Not the tires...had Michelin Pilot Sports on the SLK...Contis on the Crossfire...
Tested the fuel slosh issue in the SLK...it wasn't that either...unless there is some sort of float in there, but I only hear the noise on a warm engine anyways...not on a cold one...
SLK was autostick, Crossfire is 6 speed manual...
Catalytic converter expansion/contraction???
Definitely not the spoiler...SLK doesn't have one...