A/C question

Hey guys so I've got a question about my a/c. So when I first start the truck and kick on the a/c there is a relatively loud hissing sound from the vent area. It then quits and then it seems to hiss again (but quieter) when the compressor cycles. The refrigerant has been adjusted with gauges and is at the correct levels. Any ideas? Thanks

 

Anything I could test to find out?

 

Sounds like the normal operation of an expansion valve.

I would stick a thermometer in one of the vents and watch it for a few weeks. If the expansion valve is not working, you wouldn't cool down a degree, so I doubt it is that. By their very nature they hiss.