Have two returns for 2.5 ton furnace , one 8x20 , other 10 inch round. They both go to return plenum with 24x20 filter box.
Its very loud noise coming from 12x12 return grill (10" return duct) then furnace is on . 10" round is about 25' long with four 90ties. Take off is 7x14.
Any ideas ?
I took filter panel off - same thing. i took furnace cover to expose fan - same thing. Something is wrong with this duct?