question bout a weird noise

In my 91. at random times, usually sitting still when i hear it, and only if you slightly rev the engine, from behind the dash i get this weird sound as the revs head back to idle, it sounds like vaccum, but.... sounds like some sort of plastic/rubber diaphram is talking, sorta a "burp" sound? like a fluttering valve of some sort, anyone know what this might be? i am only worried about maybe a possible source of a vac. leak or if its something normal. Car does have AC and it works fine as does all the controls, sound only appears with slight revs..

In my 91. at random times, usually sitting still when i hear it, and only if you slightly rev the engine, from behind the dash i get this weird sound as the revs head back to idle, it sounds like vaccum, but.... sounds like some sort of plastic/rubber diaphram is talking, sorta a "burp" sound? like a fluttering valve of some sort, anyone know what this might be? i am only worried about maybe a possible source of a vac. leak or if its something normal. Car does have AC and it works fine as does all the controls, sound only appears with slight revs..

Shot in the dark time, but I would guess you are hearing the actuator for the blend door in the heat/AC system. It may be starting to leak slightly, causing it to drop the door slightly under low vacuum, then recovering when the throttle closes and vacuum builds.

I say this because the burp, or grunt noise is common to that system's door.

I say this because the burp, or grunt noise is common to that system's door.

Hmmm, guess ill have to check it out if i can get in there, only thing tho, this sound comes from behind steering wheel, best as i can judge, least the left half of the dash, from radio to the left, is that door on that side?

Can't remember on the older ones. Might be. The lines run in on the left side of the engine compartment on my '97, so it might.