2000 Nissan Maxima whining/humming noise when I turn heater

The easiest way to figure out is by turning on the heater, and check if you can hear the noise from outside of the car. If you can, then try locating the source of the noise. If you can hear it in the car, try location it in the car.

The reason the car not overheating is that you have two radiator fan motor, a low- and a high-speed. Low speed fan works independently of the heater switch. But when you turn the switch on, both fans will turn on.

If you can't hear the noise outside, it possible that your blower motor is bad, if you can hear it outside, well, maybe the radiator fan motor