Sounds like the heater flap motor as dezufo said. Iirc it's called a blend door actuator.
Had similar with mine, I 'fixed' it by pulling the actuator out and opening it up. I can't quite remember how much trim, if any, I had to remove to get to it but a bit of googling will probably see you right.
As per dezufo's post, the contacts in the position potentiometer had worn grooves into the tracks. I just bent the contacts so that they don't sit in the grooves any more and instead are on an un-worn part of the track. That was quite a few years ago and it's been fine since. Well the noise has stopped but I'm not convinced the air out the blowers is the same temperature as what it says on the dash but that might be unrelated...
If you're at all handy with tools etc then this isn't difficult, though getting the actuator out and back in was a bit fiddly.