HVAC controls "hissing"

Hey guys, any common problem known for vacuum controls leaking on the '93 Caprices? I am hearing a high frequency "hiss" whether fan is on or off and is not radio hiss either. I just got the wagon and noticed it right away and the whole ride home. I have yet to dig into her but assume that it uses vacuum controls for HVAC-Anyone hear of this issue? Do you think I am heading in the right direction?

Thanks!
Larry

It could be a number of things. A loose or broken line, broken valve, or even a bad vacuum motor.

Was able to pinpoint a blue (purple) what appears to be a / the supply line that is teed. It goes to back of fan speed selector control switch/valve and is teed to pass on somewhere (maybe to vacuum tank). I pinched off where it passes on and by initial appearances all controls seem to work at both low and high engine vacuum condition. defrost, heat, cool-floor, face etc. I can't seem to find a vacuum diagram online that shows routing clearly. Looking for any links that may show where this is teed off to (what I am pinching off).

Most diagrams mention a black line for main vacuum supply.

Thanks for any assistance- and no I obviously don't have a service manual yet!)

melon

Here is the hose diagram:

Thanks Fred!!