What can cause a indoor furnace or air handler to make abnormal noises? Here is an actual list of obstacles or issues I have found inside systems, blower compartments and duct work: Dead mouse stuck in a blower wheel! Blower Wheel is actually coming apart at the frame...
As you may be able to see from the picture, a dog chewed into the bottom of this return air plenum which is located in the garage. The furnace being located in a garage area is very common, the furnace platform or return air platform/plenum in this case is one piece....