My heater makes an intermittent whistling sound, and is really annoying me. Sometimes when I drive, it does not happen at all.

The noise is coming from the dash somewhere, and sounds like it is the blower motor. It stops completely when I shut the heater off. When I make a hard left turn, it goes away for a few seconds, but soon returns.

It happens at all speeds between 5mph and 75+mph.

What gives?