Thanks guys, RR, Robert, Mike everyone,
Glad to learn about the possibilities, I hadn't thought about a wheel bearing.

THE SOUNDS:

I recall the sound is there in both accel and decel. It's pretty loud when I'm just cruising along, constant throttle, on the highway.

Like I said above, the frequency tracks the wheel speed, not the driveshaft speed - it doesn't vary with gear shifts.

I recall very clearly that I could make the sound go way down when braking - not trailing throttle, but actually braking.

I do think the sound changes in cornering - making right turns, but not left turns. I'll double check this. I have to go get it out of storage today, so I'll pay close attention when I drive the 10 mile trip.

I'll start checking the suggestions you offer and read the FSM, I'll post back here when I can.
Thanks again, cheers,
- Art