Horrible squealing from front wheels of Prado

Twice in the last week I have had horrible metallic squealing noises starting abruptly from BOTH front wheels simultaneously. Each time I was driving on a gravel road with the centre diff unlocked. Each time I stopped and had a look - could see nothing obviously wrong (no stick or stone or anything, nothing leaking). No warning lights on dash. I pumped the brakes in case the calipers were grabbing (brake pedal felt normal), but it kept doing it when I moved off again. The wheels would turn, but the noise was loud. I turned off the engine, restarted and moved off - kept doing it. Then I reversed a bit, but it kept doing it. Then I turned off the engine, restarted, reversed again, and it suddenly went away. Drove each time for an hour or more afterwards (gravel and bitumen) without a hint of a problem.

The car is a 95 series VX 2000 model Prado, which has ABS and traction control. The fact that it was both front wheels simultaneously makes me wonder whether the TC was trying to brake the wheels? If so, where would the problem lie?

Any suggestions?