Generally electric fan motors, especially for environmentally exposed applications, are sealed units. In peaking around my fairing, best I can see, this is the case for ours, with a metal housing actually. Disassembly would compromise the seal, and in most cases, only shorten the remaining lifespan.
I'm curious about the noise though, whistling is not how I would describe a failing motor noise. Did this suddenly start? Can you post video with the noise? In my experience, there is a whine/whistle associated with these fans, but not an indication of failure.