The knob that controls how fast the heat comes out of the vents has a 1 through 4 setting but only works when i turn it to number 4 and 1 through 3 do not work at all, when i do turn it to number 4 there is this loud vibrating sound but the heat still comes out through the vents.

Any idea on what it could be and the noise comes from the passenger side behind the dashboard.