Very annoying!

On a 9-3 (1999), when the heater fan control is on the second position
(2), it make a "cricket" like noise. If I turn it to 1 or 3, it disappears.

Also, it seems to get louder when I step on the gas where you will not
hear it at all if I take my foot of the gas.

Any idea what this could be and how can I fix it?

Cheers, Ben