Hello

Near the brake booster you will find a number of vacuum hoses, the one you need to focus on is green and yellow I believe. I would try disconnecting the green yellow hose from the vacuum source, block or cap the open vacuum line with a golf tee then drive the car normally. With this line disconnected your AC system will not function pneumatically and it should default to defrost. If you do not hear the noise after conducting this test I would suggest checking the "vacuum operated" AC servos for leaks. Check the elements with approximately 4-inHg (inches of Mercury). By using a small amount of vacuum you will avoid sealing a pin-hole leak which may exist on the rubber diaphragm.

Good luck