if the motor is inside the squirrel cage is probably the bearings on the motor after 12 years...heating or cooling season/Fan ONLY... you get the noise?shut the unit off on the furnace toggle and inspect the squirrel cage for dirt build-up on the rounds if it is vibrating and free spin it to pick up any grinding or if it stops hard right after spinning it.if the motor is packed with dust from years of running that can make the whole thing overheat and dried the bearings out.if your handy as most home owners have to be,that motor is probably availible at your local Grainger
www.grainger.com supply who welcome homeowners now.if you can pull the fan section out easily take ALL the motor info Brand with catalog or model numbers/volts/frame(critical)...rule of thumb in the field new motor gets new capacitor when you go.any further questions ask away..will check back!