I have a Rheem 3 ton air handler (Model RHSA-4221-JA) with a 1/2 hp blower motor. The unit is 4years and 10 months old. The blower housing started generating a "rumbling" noise while the blower motor is running. I pulled it down and inspected the blower assembly components for possible causes of the rumbling/roaring sound. The motor turns freely with a free spinning shaft. The blower wheel and housing both appeared true and without visual damage. I pulled the bushing caps from the motor and lubricated the bushings, reassembled the blower housing, blower wheel and motor assembly, and reinstalled in the air handler. It appeared quieter; I got a new identical motor ordered. Two weeks later, the air handler once again appeared to be making a slight rumbling noise once again. I tore it down and installed the new motor. Upon reassembly, the blower assembly with the new motor was as loud as the original rumbling. The unit is pushing good air pressure through the whole house only very noisy. It might be a balance issue associated with the assembly with the motor and the wheel causing the rumbling. The blower wheel can be spun and there's no noise in the motor or the wheel. However, when installed, there also allot of vibration and the rumbling noise. Suggestions?

Pull the unit out, clean all the junk off the blower fins, it doesn't take much to offset the balance. Make sure the blower housing is tight in the frame.