Since our Crossfire is essentially an SLK320 underneath, perhaps Chrysler should investigate if below problem from the SLK320 is the issue:

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Problem: Clunking, knocking, clicking behind the passenger seat.

There is a common issue with most SLK 320s and SLK 32 AMG cars.

This related to the fuel and emission control systems in these cars.

There is an emissions valve located behind the passenger seat that can make a clicking, thumping or knocking noise.

This noise is best heard when the car is at a complete stop and the top is closed.

This issue is normal but some earlier 320s can have this valve updated to reduce its occurance.

The service bulletin # is 103/P-B 47.30/15 Shut Off Valve
Also may go by this number : 47/3045