Hissing in Vacuum reservoir area
My daughters 2009 Explorer XLT had the exact same hissing sound from behind the glove box, as described in all these posts. She also had little if any warm air blowing out of the footwell area on her driver side.
Vacuum check of the reservoir (vacuum accumulator) proved it had NO leaks!
Vacuum check of hose coming from engine to the vacuum accumulator proved it also had NO leaks. Regardless of these FACTS, the hissing was there on acceleration and just after engine shut down.
When I read the "check valve" post in the thread, I decided to check it and lo and behold, the check valve was bad! Here is the good part! My daughter is a heart cath nurse and just happened to have some a IV manifold tubing laying around her house. I noticed there was a check valve in the IV manifold, sooo....I removed it and installed it in place of the OEM FORD check valve (7B016 black & white). PRESTO...problem solved! Hissing gone and the floor heat door to the drivers side now opens all the way with good airflow. I will order the proper check valve from Ford and swap it out later.
So if there were no leaks, where was the hissing coming from?
My only explanation is this. The engine provides the vacuum source (when running) and the reservoir/vacuum accumulator stores it! As you accelerate and de-accelerate, this vacuum rate changes. When you shut the engine off, the vacuum supply is gone and the item that locks the vacuum into this system is THE CHECK VALVE. If the check valve is bad, vacuum pressure flows back and forth from the reservoir/vacuum accumulator AND THAT IS WHAT YOU HEAR AS A HISS. NO LEAK, just FLOW! When the check valve works properly, it locks the vacuum in the system and prevents any air flow back to the engine manifold! With the vacuum locked in the system, it also holds that air flow door to the drivers feet in the wide open position. With a bad check valve, that door will not stay wide open all the time, only when running a high rpm~