I'm pretty sure that it's not the speaker. I turn the volume all the way down and it's dead quiet all the way through the start-up, running and shut-down processes. Also, the humming doesn't happen all of the time. So, I think it's probably the motor or the trim like you suggested. I'll try those. I don't think it's the gears; they're nylon. I didn't think that they needed a break-in.

Thanks.