If you pop out the glove box liner and reach down along the right corner of the dash, you’ll find the vacuum switchover valve. It’s a complicated little gadget consisting of a gang of electrically controlled vacuum valves. The valves adjust the vacuum level to the various control pods in the heater box. It’s tough to diagnose, and it could be a bad vacuum pod, bad switchover, bad head unit, or a simple vacuum leak. And then someone’s they just get noisy with age. You’ll have to hunt for the HVAC manual for your car for more specifics.