I drove my car last night and no problems.

This morning when i woke up and started the car there was a hissing sound coming from the dash... it sounds like a tire with a puncture hole hissing...

The fan was OFF. The heat was OFF. The selector was set on off.

it doesn't matter if i turn the fan on or off

Here's the strange thing. If i put the selector on "defrost" the hissing noise goes away. Any other selection, even off and I can hear the hissing noise?

WTF?

My first thought was heater core? But the heat seems to be working...

Help???
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there is no dana only zuul

the boost gauge has a vacuum line with a tee in it that breaks all the time. if the boost gauge is inop also, i'd look there first. should be right above the driver's right knee. good luck, chuck.

Sounds like you have a vacuum leak in the dash. The vacuum comes from the engine intake, so it doesn't matter whether the fan/heat is on or off, but vent position does.

Interesting that the problem goes away in defrost (which is the default position). Does the hissing problem get better/worse when acceleration vs. cruising on flat roads?

First step would be to check the condition of the check valve coming off the intake manifold. Doesn't sound like the problem, but it's easy as pie to check. If that's not the problem, it might be time to dig into the dash to find a leak.

-Ryan
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Athens, Ohio
1987 245 DL 314k, Dog-mobile
1990 245 DL 134k M47, E-codes, GT Sway Bars
1991 745 GL 300k, Regina, 23/21mm Turbo Sway Bars

someone also suggested the vacuum actuator...

the leak seems steady regardless of throttle position... also not hissing in the floor/dash setting but leaks in both individual floor or dash setting... (face and feet)
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there is no dana only zuul

The heater air redirecting flaps are vacume operated, you have a vacume leak, maybe on the line going to the defrost actuator. you will have to investigate inder the dash.
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David Hunter

My 1990 740GL had the same thing. Problem was a broken vent control switch. Its not hard to diagnose if you can pull the small control panel from the dash and look at the back of it, but can be difficult to repair depending on the exact reason for failure.

Mine had a broken plastic stud which holds the switch in place. I repaired with a small pan head bolt and epoxy. After a couple of days one of the vacuum lines came off and I had to take the panel out again to put the line back on. Its been several months now and no further problems.