2004 Nissan Frontier Whistling

Hi. After some time of sitting idle, my truck whistles like an overheated tea kettle immediately upon starting. This noise continues for about 5-10 seconds. When it is colder outside, like during the winter my truck would whistle longer up to a minute long. At times, when I am stopped at a stop light or I switch gears/accelerate I hear the whistle. Yesterday, I took my truck to Brake Masters and a mechanic examined my belts. He showed me he was looking for visible frays or small fissures on my belts and he couldn't see any. He said everything looked find and I could drive it longer. Why is my truck whistling?

P.S. I already donated $5.00 via first prompt (red exclamation point) but I don't think my question went through:
Receipt Number: 2274-5095-1821-5798

Thank you.