[Hajimasabi]

My 2002 Rav has 250K mile son it. On a trip across NV yesterday after driving for 7 hours, we started experiencing a noise upon either accelerating to pass or to maintain heat going uphill. The noise sounds and feels almost exactly like a downshift but is accompanied by neither an increase in speed or RPM. It sounds like the engine/trans is wound up but the tac does not move at all, holding steady around 3K. When I let off off the gas, sometime sit would cease, but a few times it stayed "wound up" almost until the car came to a stop. I tried shifting into neutral and turning the OD on and off while the noise was happening, but neither seemed to make any difference. It does not sound like a fan issue or a belt/bearing. No metallic squeal or similar sound, just a "revving" sound and a vibration thought the steering and gear shift like a solid downshift.

Any ideas what's going on?