I get some low humming at 700 RPM idle coming from the front, in neutral or D [foot on the brake]. Had timing belt and water pump replaced recently. Current mileage ~309,000. Any clues welcome. Thanks!

Potentially your alternator. Get a stethoscope & take a listen. Could be a bearing inside going bad.

Thanks, I will try that!

I recently discovered that the humming my 4Runner was experiencing was in fact my alternator. I had replaced my OEM one a few years ago with an aftermarket one and soon enough it went bad. It was an absolutely awful noise and I couldn't figure it out for the longest time. But eventually I went with a good buddy of mine to visit his mechanic, and his mechanic listened to my alternator with a stethoscope and sure enough you could hear that bearing inside of it. So fortunately enough I was able to pick an OEM alternator up from a 99 at a local junkyard for $20.99... let us know what your results are.

Also: does the pitch of the whine change when you turn your steering wheel? How about when you accelerate?

Yes, it turned out to be the alternator, which was still under warranty -- no cost! Thanks for your help!