Why is my car's heater screaming at me?

The heater has been making irritating noises for the past year or so, at first intermittently, but now it's getting worse. I notice now when the heater screams, the heat fails to pour out as good as it should. At first I thought something had gotten caught in there, as it sounded like a piece of paper flapping rapidly, but now it is fairly screeching. Could it be the blower failing, or perhaps the intake air vent is clogged, as was suggested to me? If the latter, where is this located so I can unclog it?