Could very well be the differential mounts/bushings-get them checked before you pop for the bearings. The diff bushings made the same sort of noise, but only when applying the throttle; ease off the throttle, pop it in neutral & see if noise goes away.

In my experience, when these bearings go, they get real loud real quick. Good luck Neil!

PS I doubt there are HD bearings-the ones in there are huge!

__________________
"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo