Hi Dale,
I also changed the motor of in my heater blower, quite a while ago, with one out of an wrecked late 1960s Oldsmobile Starfire Convertible. With the original motor, I could breath heavier then the air coming through the defroster vent. Since the Olds installation, my defrosters move so much air that they whistle, even with the heater vents open.
When you changed the motor, did you install the squirrel cage on its shaft? If so, how? Depending on how you positioned the motor and squirrel cage, you may be rubbing the inside of the surround. When I installed the Olds electric motor, I could not just replace the motor but needed to create an adapter out of a piece of metal to allow movement for correct placement of the squirrel cage within the surround.
One installed and after validating its operation, I trimmed and painted the full unit and now you can't tell there is a optical difference.
Hope this helps,
Ray(64BJ8P1)