Hissing with Defrost

[nick0789]

I've got a 2005 Z71 Tahoe (~150k) and just noticed a hissing sound this morning shortly after starting.

I cranked it and turned on the defrost and let the vehicle warm up for a minute or two before getting going. I was backing up and as I turned my steering wheel to the left I noticed a hissing noise coming from the passenger side (I'm assuming the floorboard area).
Turned off the defrost and the sound went away. When I turned it back on a few minutes later and the defrost fully kicked on full blast the hissing sound came back. I am not currently noticing it while driving straight, only while the steering wheel is turned.

I wasn't able to find any directly-related thread on here, only ones about the AC, so I'm looking for ideas.

Is this some actuator door issue? I don't have the old service records right in front of me at the moment, but the previous owner had some similar work done on it in its first couple years (2008ish) so I'm wondering if it's a repeat problem.

Thanks!

 

[PG01]

Does everything blow out the correct vents which you select? Hissing is vacuum operating the actuators.. it may be disconnected from an actuator, a hole in the rubber/plastic vacuum line to/from actuator or controls, actuator going bad... or just age and you’re simply just noticing it now. Many cars ive owned have ‘hissed’ at me but everything worked correctly.

 

[nick0789]

Does everything blow out the correct vents which you select? Hissing is vacuum operating the actuators.. it may be disconnected from an actuator, a hole in the rubber/plastic vacuum line to/from actuator or controls, actuator going bad... or just age and you’re simply just noticing it now. Many cars ive owned have ‘hissed’ at me but everything worked correctly.

Thanks for the response.

I checked the flows (hot and cold) during lunch and they all appear to be working normally. Only questionable one would be the defrost on the far right set of dash vents, though I'm not sure if it's just a flow design thing.

I've just not noticed it in the past several months, even in colder weather, so I wasn't sure if I was starting to have a problem or not.

 

[M1Gunner]

Our hvac systems don’t use vacuum for operation/controls. The hissing sound also may be a sign that you’re getting low on refrigerant. When you turn the heat/defrost on it also turns on the ac because the evaporator in the dash will aid in reducing the moisture inside the cabin to reduce the fog on the windows.

 

[PG01]

Maybe he's got snakes under the dash then..