Engine AC unit makes hissing sounds off an on

[Bobc]

Hi all,
I have a 2003 Sun Voyager 8368 with a Workhorse Chassis and when i am driving my vehicle ac ( not roof units) will make a hissing sound about every 20 -30 seconds like it is decompressing? All works great, It supplies cold air thru the dash vents buy just wondering what may cause this and a fix? I just bought this a few months back and i believe it sat for a few years prior to my purchase.

[Bobc]

Recharged it to 37 pounds and now works great