Just a thought, it has happened with mine a few times
If you have parked under a tree and small leaves or pine needles have gone through the vent in front of the windscreen, they can get down to the fan and rub on the blade making a wirring noise, or sometimes a tick. Generally playing with the speed, changing the air from recirc to fresh and back a few times can sometimes upset the airflow enought to dislodge them. They leaves or debris can also be picked up from inside when you get in with dirty boots etc. The recirc air vent has 2 pickups on mine, one at the top behind the dash and one at the bottom right at floor level. I have now installed airconditioning filter wool over the inlets to stop the dirt and stuff entering the evaporator.
The other noise it could be is the refrigerant flow through the TX valve which can make a wirring sound, and the ticking sound when you sre on heat can come from the thermal expansion of the heater core, just like exhaust or engine heat clicking. although it should generally last only a few minutes if it is that.