99 Tahoe "sucking" noise behind dash

[spicewood1]

I have 2 99 Tahoes. One has the stuck in park issue on another post. This one has a vacuum noise coming from behind the dash. Seems to fluctuate with engine vacuum. Sounds exactly like an old Qjet at idle with the air cleaner off. It is very annoying. I have disconnected everything trying to isolate the sound with no luck. What all is behind the instruments that could make that sound?

 

[correll20]

The air vents use vacuum to change the direction they are flowing!!

 

[taylorjm]

Maybe it has the D-69 swedish suck machine option? I was going to get it but then why would I need a wife? I'd spend all my time in the truck!