2001 Lexus RX300 cold engine squealing/whistling noise

My father-in-law’s 2001 Lexus RX300 makes squealing/whistling sound when engine is cold. Multiple serpentine belts replacements and adjustments made no difference. One mechanic said it’s the bearing inside the alternator, but I find it difficult to believe. Why would bearing sound go away when it heats up? Besides, I’ve never ever heard of or seen anyone having issues with bearings inside things like alternators and such.

Usually when a bearing begins to fail in a component driven by the serpentine belt and starts making noise, the serpentine is removed so each component can be rotated by hand to see if there’s any roughness in the bearings of said components.