Agreed, but then alternator belts will normally slip and squeal on engine accelleration as well.
The fan on the heater dosent pull that much current. The biggest drain is the heater rear window, so see if you get the squeal by turning that on instead. If you do then probably the belt if not then look for something else like heater motor bearings
it used to sqweel ... and the sound used to go up and down with acceleration. I got it sorted yesterday and as i drove out the belt on it slipped ( or summit like that ) and made one hell of a sqweelin noise ... but i took it back and he tightened it up. sorted
btw bearing for the alternator ( if thats what it is ) cost about 40 quid