That is weird.
If I put my heater on HI, the power steering belt will screech insanely loud. and is deafning when I turn. Heater on LO, ..... not a peep.
Sounds like you have a bad connection in the system that is making contact, and then not. Perhaps like when you are hooking up speakers to the back of a home stereo and it crackles with sound for a sec and then silences.
The heater switch is controlled by cables if I remember correctly so I don't think the switch will go bad like a pwr window one would.