When I first got my van the VW Jedi told me that almost every one that he'd
worked on had a broken motor mount on the driver's side... mine is. The
engine doesn't appear to move when looking at it, but if I "bench" it from
below I can see it. I've also had a transmission mount on which the forward
rubber was breaking down. Perhaps the drivetrain is shifting slightly when
you let off the gas.
Cya,
Robert

'87 & '86 Auto GLs