When I shift from park to reverse there is a metal clank sound like two steel plates clinking, that can be heard with the windows open. Crazy loud in a parking garage! INot sure if it is transmission or drive line. Other than the sound, the car seems to drive normally.

Is this trouble? Any ideas?

 

Is there any clanking noise when the car is driven slowly over sharp bumps? There are several fasteners (screws) used in the motor and transmission mounts. They are designed to fracture during impact - so the engine and drive train can submarine under the car instead of intruding into the passenger compartment (cell). However, these "screws" (actually very thick bolts) sometimes break in normal use. They might be clanking around under the car. Pull the plastic shields and see if any have broken and fallen out and if the engine and transmission mounts are secure.RealOEM.com BMW E60 530xi Engine Suspension

It's part #8 in this diagram - your model may be slightly different.

 

inspect the 'shifter linkage'. That is the connection between the shifter and transmission.

 

With wear and tear they break inside the block/trans? Maybe check the drive shaft U-joint.

 

I has this same issue once on a 2002 530i. Drive shaft/joint.

 

Is the car on a slope, ie nose facing a downward slope? If yes, my car always does it, I guess because P is really engaged preventing the car from moving fwd. If, however, on a totally flat surface, it does not happen to me.