Tried posting this on Rennlist without any luck.

Last year I replaced my AC refrigerant pressure sensor after it let loose and dumped a quarter of the gas to the atmosphere. $50 part plus refill. Following the refill at a reputable shop I have had an occasional noise that I can best describe as pressure being relieved or balanced.

I'm curious if this is normal or if it's possible the refill procedure was not followed properly. The service manager said it's an easy/straightforward process to refill but another person said it was a complicated process requiring PIWIS to open check valves. Which is it?