1990 Toyota 4Runner, V6, automatic transmission, 2 or 4 wheel drive (which I always keep in 2WD). I've owned the car for about two months, the previous owner, whom I know and trust, drove this car for a year without any major troubles.
Update 2012-10-22 - It happened again this morning, on a 55 MPH highway. I think it's a pump or the alternator, because it sounds an awful lot like this YouTube video, but a lot louder. It seems to go away once the car has slowed down enough for the motor to go below ~1000 RPM (unless it's something that spins in direct proportion to the wheels, as opposed to the motor). Needless to say, this makes positively identifying the source of the noise difficult. I guess the question now is, "what other than the alternator can make this kind of sound"?
I was driving on the interstate, and after less than two minutes, a loud noise started in the engine compartment or dashboard. It sounded like a giant fan straining to suck air, or a motor spinning at a million RPM. The noise didn't fade immediately when I took my foot off the gas, but it seemed like it got quieter as the wheels slowed down - it was gone (or almost completely gone) by the time the wheels stopped. I got into the breakdown lane, shut off the engine, checked under the hood, and there was no smoke, unusual smell or anything falling off. I restarted the engine, looked under the hood again, and everything was running and sounding fine. I got off the interstate, and finished driving to work on the back roads, and the noise did not return. Steering, acceleration and braking were fine during the incident. I drove it back and forth to work the next few days, including on the interstate, but it doesn't reliably recur, and it always goes away by the time the car comes to a full stop.
It may or may not be related, but I just replaced the steering gear box and switched from 33x12.50-15 to 31-10.50-15 tires (previous owner took it offroading, but I'm not into that) two days before the first incident. After replacing the steering gear box and tires, the steering seemed tight for a day, but that got better rapidly on its own. As I said above, during the loud noise, the steering seemed fine and responsive while I was coasting into the breakdown lane, which is why I question whether it is related.
So, any ideas? It sounds like bearings dying in something that spins (belts can't make that noise, can they?), but since the noise is gone by the time the car comes to a stop, positively identifying the source may prove tricky.