... Here's an interesting phenomenemenen...
We were summoned to check out a "loud, noisy" rooftop downblast fan at the Diner we do alot of business in.. the thing was making an awful racket,..and we shut her down to dis-assemble the mushroom housing..
We found nothing loose or rattling inside so we put her back together again and I hit the switch,..and all that happened was this LOUD (the best way to describe it is like a hum) or more like an electronic buzzer at 100db.... the fan is a direct drive fan, and all the connections were good.. it seems that the negative air pressure at the roof penetration forces the fan to "free-wheel" in the opposite direction, and the start windings are shot.. is this the case??
.."if it ain't fixed,don't break it...call a Licensed Electrician"