2000 blazer hissing noise

hello, i have a repair question i was wanting some help with. i have a 2000 blazer 4wd that has developed a "hissing noise" that sounds like it is coming from the dash board. i can select different cooling modes and vent configurations and that does not change it. yet upon acceleration the "hissing" goes away and if i let off the gas it comes right back. is there a vacum leak behind the dash board?

any info would be appreciated.
thank you

It definitely sounds like a vacuum leak. Look under the hood for the plastic vacuum ball and make sure the hoses are still connected. Follow the hoses back to find the break or disconnect. I just got a 2000 myself and will look to see if there is anything in the dash that might be disconnected

There are several, (4?) vacuum actuators in the dash, some have two vacuum lines. Could be the mode switch also, but not likely. Make sure all vacuum lines are connected tightly. Stick your head under the dash and you might be able to tell the general area, then get the flashlight out and start looking.