Mercedes: Tapping noise, torque converter, neutral conditions

Question
Good evening Rob, have a Merc 180k auto,2006 rhd. following a drive at idl there is a regular tapping noise that is audible only inside the car, noisier in the boot,been back and fourth to mb dealers (there now) we induced and held the noise by engaging reverse gear 950rpm and foot on brake, otherwise noise heard when reversing into parking space... any thoughts?

Thanks, David  

Answer
Hi David,
It is curious indeed. You may have a pump in the transmission or torque converter part that is loose. As I understand this tapping, it is not present in any forward or neutral conditions and only when in a still position. It appears only during engagement of the reverse bands. Yes?
If you have 180K on this transmission, then I would say you have been blessed with longevity without too much maintenance.
Thanks
Robert