Heater blower/fan high pitch whine

I am hearing a high pitch whine from beneath the glove box when the heater blower is turned on. The sound is not heard when on highest speed (probably because the fan is making too much noise).

I just replaced the motor/blower thinking it was a bearing going bad, but it did not help. The sound almost reminds me of the whine from a car stereo amp that is poorly grounded.

Any thoughts on what the problem is?

Thanks!