I have read archives and discussions on this yet no definitive solutions or answers - is there a tsb for crossfire? The 320 seems to have one (see below).

It occurs only when shifting into 1st or reverse.

I had my generic mechanic take a look and seemed to think it was loose bushing on the rear axle mount but then I am not sure. The bushing seemed to have play but could be intentional.

The following is from mercedes 320 slk service bulletin:

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