Overdrive noises?

I've used this forum considerably lately, and with much luck,
so I figured I'd try all you experts first on this as well:
I'm detecting a sound I can only describe as a "warble"
or some kind of drone, from the middle of the car, or the tranny.
I think it only happens in overdrive, as it's not there at any
speed in 3rd gear, and goes away when in Neutral. So around
50-55mph, when just cruising. Any ideas? I just had the AWD clutch plates
fixed, and haven't had AWD for awhile...if that helps. TIA!
Yes, I am due for new tires. And no, it's not a wheel bearing.
(Rebuilt tranny with 43K)