It could easily be a water pump, generator bearing, or similar part that starts squealing after things warm up a bit. But you've eliminated the water pump and generator by removing the belt) The shaft of a bearing that's worn may start "walking" around inside the bore, (chattering from side to side) and cause the squealing. A bump can cause that to start and another bump could stop it. Is it speed related (change pitch with different speeds)?
Anyway, my vote is a bearing. I'd check the front wheel bearings first to see if they're binding up, possibly causing the inner race of one to chatter on the spindle.
