Bird/Monkey Sound in my 2003 Camry XLE

I randomly hear a very loud sound that I think is coming from under the hood, but it is hard to tell. This sound is something you may hear high in the canopy in the Amazon rainforest. Maybe a bird or monkey the starts at a relatively high pitch then plunges to a lower pitch. No mechanic believes me. My guess is that is is produced by one of the systems under a vacuum, but I can’t seem to reproduce it at will. Many times I’ll just be sitting still at a stop light and it happens several times. Please help me identify this sound.

A loose belt would seem like a likely area to check.