Weird Noises Before My Central Ac Shuts Off.

by Stephen
(San Antonio)

My central air conditioner makes a loud sigh/hiss like air escaping or valve opening about 60 seconds before it turns itself off.

What is this and are parts going out? This did not happen before and has been serviced since but they say they don't see a problem (they came out twice and it is now getting worse!)

They replaced some switch/unit that prevented it from going off as it was forming ice around the pipes. That's fixed but the noise remains.