On Monday, January 6, 2014 8:30:58 PM UTC-6, Bob_Villa wrote:
This is an NG unit...I pulled-off the exhaust PVC and looked at the plastic fan...it's not loose and no weights or scratches where they might have been.

The gas shuts-off 1st and after about 10 sec. power to the inducer motor is cut-off and then there is a shutter to the whole assem. (jpg link).

Maybe it's caused by the long PVC exhaust run (about 20')

It has been running well in this -15 deg weather...just wonder if anyone has heard or seen this...

I lubed the bearing of the fan housing and this quieted it down...it looks like a sintered type.