2007 Hissing Noise

[ltznc]

I have a 2007 LTZ that I love, however, when I'm parked/stopped, there is a hissing noise in the back (I think it's coming from back right where the cover grille looking thing is). It's really high pitched, and sort of sounds like air escaping rapidly. Sometimes it's quiet and sometimes it's loud, but always noticeable. Sometimes I hear a noise coming from the front before it starts (not the same noise, sounds normal and coming from under hood), not sure if there's any correlation though. Has anyone else had this problem/know what it is?

 

[MO Viet Vet]

May be hearing a noisy fuel pump.

 

[Nick H]

I hear something similar in mine also (2008 LTZ), but minus the high pitch. I'm thinking it is probably something associated with the temp control. I am usually always running my defrost/floor setting during colder months and I know that those systems use some sort of pressurized check valving to regulate heat. But in my LTZ it can only be heard with all systems turned off and while I am at idle and not moving. It kinda sounds like someone or something exhaling slowly every 10 to 15 seconds.