How to get to inside air vents

The adjustment that changes from defrost and heat are adjusted by a wire control that is inside a casing. Look under the dash where the heater control and follow it to the heater box. While you move the controls up and down. The end of the wire is formed a O that is held on by a clip on each arm. Sometimes they get bound up or even at times pop off. If you can not move it then take the wire off and use so lube on the wire also move the heater controls back and forth to make sure they are free. Once evertyhing is free then reassembly.

There is a vacuum line that runs into the control switch. Then to the function control. Some areas they are made of a plastic and some are a small rubber hose. See if you can pinpoint the sound. If the plastic line has split or broken you can use a patch it with another rubber hose putting it inline. Some of those plastic vacuum lines are a pain when they become brittle.