My 2005 MCS Cabrio has developed a rattle in the boot or passenger side rear wheel well that is making me slowly insane. It happens only at low speeds and sounds like a very loose object inside a plastic housing. Today I took out the rear seats and the passenger side wall but did not find anything. And it still happens with the seats and wall out. I also checked that there's nothing loose in the battery well. Anyone seen this before?

 

It could be the heat shield around the muffler. Mine rattled for a while until I bent it out of the way. Not sure how it got close enough to rattle in the first place though.

 

Try your Wheel Arch Trim...check under neath in the wheel well or check to see if a piece came off the clips.

 

And look at the trunk latches....quick way to test out it is not the latch...put one or two wraps of electrical tape on it... Shut the trunk, and see of the rattle is still there. I know on the regular cars, the chrome latches were replaced with black ones to solve a rattle problem...I think it was that way...sure someone will chime in. I assume that the latch on the cabrio could be loose in the same way, making a similar rattlt.

 

I put a wrap of electrical tape on the chrome latch in mine. Fixed the annoying rattle that was there forever. It is not a Cabrio though. I do not know how their latches work.

 

Mine does that too.
I'll try the electrical tape.