My g56 with 170,00 miles on it has started, for lack of a better description "Rattling" when i let off the accelerator pedal in 6th gear at highway speeds. If I put pressure on the shift lever it seems to go away. It has also slipped out of gear a couple of times when at speed and suddenly let off the pedal. I can't get it to do it when I've tried to make it happen so maybe it's in combination with hitting a bump.
This "Rattle" is different than the expected Gear roll over noise I picked up when the new South Bend clutch and flywheel was installed. Any Ideas would be appreciated.

 

I would drop the oil through a filter paper and see what metal is in it. My son's 2011 G56 started to make noise at about your mileage and it was completely toast by a little over 200k.