1987 740, M47 swapped from AW70.
Ever since the swap, around 1900 RPM when coasting and foot is off the throttle there is a prominent low frequency vibration coming from the rear/middle floor section. Put the foot just a bit on the accelerator, all goes away. Most noticeable in 4th when doing 60 at 1800/1900 RPM. Goes away before 1800 and after around 2100.
-New center support bearing
-Newly balanced driveshaft
-New motor and tranny mounts.
Not sure what it could be... It's driving me nuts! It's not severe to the point where the car is undriveable but its very annoying and definitely shouldn't be there.