Originally Posted by andrewwynn

Same reason that worn CV cause vibration or engine mounts. When engine is running and power train is engaged the individual hits of power load the system then briefly relax. When there is a worn part, the output shaft of the engine gets to twist things a little farther before the slop is removed.

Watch how much play is removed for example when a car is shifted from reverse to drive. It's the same play that is coming into action.

Often (likely always) when the guibo is the source it's loose bolts and you can punt on replacing by torquing them down.

The drive shafts are torsion springs and will store plenty of energy even at idle to push the transmission slightly backwards to loosen up the drive train ever so slightly to set up for the next power push and following jiggle.