My vote is for the blower motor going out / bearings squeaking.
Here is why... A change in AC Blower speed changes the chirp which means the 'blower motor'. If it was the belts or AC Clutch then the force of the air blowing into the cabin wouldn't change the pitch of the chirp cause the belts & clutches spin at RPM speed not blower motor speed.
There is a great DIY on blower motor replacement! Good Luck!
2004 BMW 330Ci Convertible