94 s10 ac/heater controls

i just purchased a very well maintained 94 4x4 blazer. I tried the a/c controls to make sure it works and it worked great. However, now i hear a hissing sound coming from the control panel. The hiss goes away when the selector is in max a/c or bi level a/c. Anyone know what this could be before i tear apart the dash?

Welcome to AF
Usually a Vacuum leak.
Peak under there an see if you can spot a vacuum leak.
May be just a hose off.
Good Luck
MT