heater control
Melting Slicks
Joined: Mar 2008
Posts: 2,023
Likes: 6
From: Austin Tx
There is a vacuum actuated diverter that controls whether the air flows up to the diverter, or down to the floor. It sounds like the vacuum line to this diverter is disconnected, and that's your 'hiss' sound.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Problem is likely on the heater control head in the tranny tunnel. There is a vacuum distribution manifold on the left side of that control head. The seal on that manifold goes bad and/or there is some problem with the hoses. The Chevrolet Chassis Service Manual has all the info you will need to troubleshoot, disassemble and reassemble that HVAC stuff.
