I live in Florida and several days ago on a cool 63 degree day I decided to turn off my A/C and use the vent. The air blowing out of the vent was quite warm which didn’t make any sense given the outdoor temperature. After checking the temperature controls and pressing the recirculation button on and off a few times, I gave up and turned on the A/C. Three days later I started the engine and heard a very audible and annoying tapping sound coming from right behind the dashboard. I turned off the radio, the fan, the A/C, played with the knob that adjusts the air to the floor, dash, windshield etc, and played with the temperature control knob all in an attempt to stop the annoying tap, tap, tap. It finally stopped when I pushed the recirculation button (the one that recirculates inside air as opposed to outside). Upon repushing that button it took a full 5 seconds but the tapping started again. The tapping is constant about two taps per second and does not speed up or slow down or get louder or softer with anything the engine does. I had my husband listen to it and the first thing that he noticed as he sat in the passenger seat and listened to it while the car was idling in the parking lot was that the taps were in exact sequence with the blinking of the “fasten seat belt light” for the passenger. My A/C seems to be working and I can stop the noise by keeping the recirculation button pressed but am I ignoring a potential serious problem? This is the sunshine state. I can’t live without my A/C but I can live with this button pushed to avoid spending big bucks to have my dash torn apart.
One of the “blend doors” in your HVAC system is malfunctioning. Have your mechanic check underneath the dashboard in order to locate the offending part and your problems should be resolved.