> That's a thought. This gearbox has given me trouble in the past.
> When starting up the car again, I will try what it's doing under the
> conditions you describe.

Does your car have a gear or chain driven cam?

A chain could also produce a "bouncing" between
"front" of the chain pulling against normal
resistance and "rear" of the chain pulling
against the inertia to slow the rotation during
dropping RPM.

Also, if the chain were stretched or the
tensioner weak on dropping RPM the "front" side
of the chain could loose and be flapping or
banging around.

Again the condition would be dropping RPM (as
opposed to constant RPM with engine braking).