Heater noises

I have an 84 full size Bronco, 351W. The heater makes an awful "growl" or deep "squeal" when its running. There is warm air coming out, although it does seem somewhat low in flow. The trouble seems to be from the position of the selector, not the fan motor. If I turn the fan hi or low, the sound stays the same, but if I turn the selector to "off" it goes away. The noise is somewhat intermittent, but mostly there. It does go away at times for a few minutes. Seems to be worse the colder it is outside.