Why would there be a squealing noise from under the dash?

My guess is a turn signal flasher, I think it is part # 552 or #323. It is the diameter of a quarter and about 1" long. usually silver in color. Locate it when it makes the noise to confirm it. If it is buy a new one, usually a few bucks. If that is the problem... Flasher? Nope! This'll be a speedometer or a speedometer cable! If the noise is present when the car is motionless, my theory is blown, though. Couldn't it be the heater fan?