The most likely suspect is worn bearings in the fan motor. That doesn't hurt anything but it can be irritating.
If the noise is more of a higher-pitched whistle, that can be caused by air leaking from a seal in the heater box or past a mode door that isn't sealing when it's fully closed. The clue to a leaking mode door is the noise will change when you switch between feet, face, and windshield vents. The clue to a noisy motor is the noise will change when you switch speeds. The pitch may not change but the loudness usually does. All of these noises will occur with the engine off or running.
If the noise only occurs with the engine running, we have to look under the hood, but then the additional things to consider are the noise will still be there when you turn the heater off, and in some cases, if the noise is gone with the engine off, it can still be due to air leakage in the heater box. System voltage will be higher when the engine is running, and that makes the fan motor run faster. That may be just the little extra that's needed to make an air leak whistle.
Also be aware that on a lot of GM cars they have it wired so the heater fan always runs, even when you have the system turned off. If motor bearings are the cause of the noise, it could be there anytime the ignition switch is in the "run" position, whether the engine is running or not, and whether the heater is turned on or not.
Thursday, February 20th, 2014 AT 8:33 PM