I just purchased a 1991, automatic, Toyota pick up with a rebuilt motor approximately 3,000 miles ago.

When I start it cold if fires up fine but makes an odd noise. It is like a chirp that seems to be coming from under the cab. When I load the engine it accelerates in frequency and then goes away. Then when I put it in reverse it will chirp for maybe 30 seconds just idling or until I accelerate and then let off the gas.

The transmission was recently serviced and I am certain it is not a loose belt.

Any thoughts?

Could be a bearing in one of the idler pulleys, or water pump, PS pump, alternator...