A/C hissing inside car

[G8RB8T]

Lately my 2013 explorer occasionally "hisses" when the a/c is on. A/C seems to function fine, but not sure what this could be other than maybe an a/c leak?

Possibly heater core?

 

[03snkvert]

Wifes was doing the same last week, and then ended up not working. Ours was just low on freon!

 

[kmarnes]

Could it be as simple as one of the vents is nearly closed?