My first guesses would be alternator noise in the radio or the fuel pump. That dont really sound like either and they wouldnt change by pressing on it. Next guess would be the heater/ac fan, but you said it runs fine ...except it dont blow out the floor. It might be the 'motor' or relay that moves the distribution door. You might try to see if you can force the door that directs the heat to the floor to move one way or the other. Something might be stuck in the opening, jamming it. I know for a fact mice love to crawl into car heater boxes.
Yea, the first thing I thought when I heard it was alternator noise in the radio as well. That has been ruled out, as well as the fuel pump. The pump comes on for a couple seconds when you turn the key on, and turns off, as it should.
There is a relay that controls the door to direct the air to defrost, floor, vents? That could be the culprit. Does anyone else know for fact that the relay is located in that area, and that it could make that noise?