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Hissing sound comming from under dash

91 chevy S10 occasionaly makes a air hiss somewhere around the steering collunm.usually goes away after driving a while.suspect its something to do with the heater or AC.When I turn off the engine the hiss gradually dies down.

It could be a small vacuum leak. I’ve owned two S-10s (a '95 and a '96) and both of them had vacuum-actuated blend doors. Not sure if that’s the case on a '91.

If it was A/C related, you’d hear the noise only when the A/C is operating.

Thanks: I suspect the doors because the hiss changesfrom heat to vent.quite annoying but it doesnt seem to be serious…

Yea it sounds like a vacuum/blend door issue. Maybe a loose or cracked vacuum hose. I don’t believe it is too difficult to get to those on a S10. Usually the repair is $1.00 parts and $250 labor. Good Luck

I am wondering if this might be a problem with the brake booster. Check to see that after the ‘hiss dies away’ there is no assist to the brakes. If the booster is storing vacuum correctly, you should be able to apply the brakes two or three times with assist after the engine is turned off.