[EVRLET]

Since the blower motor operates for both heat and AC, have you tried turning the system on in the heat only mode? Does it go away? Those motors do not fluctuate with engine RPM.

You can remove the motor, hold it in your hand tightly, turn it on and know for sure if it’s the motor or not. Or grab a hose and place one end on the outer motor housing and the other end on your ear. Turn it on and see if you can say yep, the motor.

I can’t think of anything in the interior compartment that fluctuates with engine rpm. Maybe electrical noise through a poorly grounded stereo, but that’s it.