> It has been running exceptionally well, very smooth until just
> yesterday when I first noticed a sort of nondescript rumble/vibration
> after a long descent at relatively high speed, from 17,500'.

Erik:

I had a somewhat similar vibration one time. It turned out to be a
loose flap of anti-skid tape on top of my wing. Look for any
thing loose that should be tight. The fact that your problem is
airspeed dependent suggests an air-driven resonance. Could be
anything from a control surface to a loose piece of baffle gasket.

-bob