Clunking noise!

Check your muffler mount, willing to bet it's bent and the muffler is hitting part of the unibody (up in the cavity where the muffler is tucked up into).

I noticed mine to be bent, when straightened it only took a couple of months to bend again

Danish said:

Hey MGD, is this the muffler mount?
Mercedes Clk430 Muffler Hanger at Auto Parts Warehouse

Click to expand...

That is part of it. The part that was bent throwing my muffler off was the rod that comes off the muffler itself and into the rubber. Not sure why mine was bent but it was and it would cause the muffler to clunk against the cavity. It's located to the left top of the muffler if you are standing behind the car looking at it.