Hi !
This is my first posting here, Unfortuanately it has to do with a problem with my 1999 Silverado.
I keep hearing a hissing noise from under the passenger side of thedash every time I use the AC or deforoster. It sounds like an air leak like someone letting the air out of a tire. It is not a constint noise but it happens every 10 seconds or so, really as I have counted... one thousan one.. one thousand two etc. and it occurs every 8 to 10 seconds. The sound slows up after about 10 minutes or so of hwy driving and sometimes stops altogether.+The last few days I've heard a louder hissing noise after my truck starts up then it starts the hissing noise again under the dash. What gives?? Its really driving me nuts.
Thanks!!!!!

 

Sounds like a vacuum leak at the air door! I could be wrong but if you are sure it is in the cab it shouldn't be that hard to diagnose and repair it.

 

99's do not use vacuum for the doors...i imagine a door is not sealing off....try playing with the recirc button and such to see if you can find which one it is

 

Another developement is that I notoiced that it does not make any noise when the truck is idling. You have to kick it up to about 1500 RPM before it starts . Trouble is that I can't see any thing in the unit as of course it is completely enclosed. I tried switching between the various settings on the controls and nothing happens that is, it does the same thing. Another thing is that the sound dimishes as the times goes by. It is the noisest when it is first turned on. Very perplexing.

 

ok...i'm thinking the evap core may be flexing in the case...but..i dunno...I'd have to hear it...

look at this...make sure the antenna grommett is there...on the rh side...if you have a hole to the outside,it could whistle in...I've seen that

 

Nope antenna grommett is in, besides it makes the noise when the truck is standing still and the RPMs are brought up to 1500.

 

the heater hoses or a/c hoses must be buzzing the core..maybe have someone hold it at 1500,and wiggle the lines under the hood and see if she changes

 

Ok, an update here. The noise seems to be coming from under the dash but actually is in the engine compartment. Can't seem to locate the noise as it sounds like a valve is opening and releasing a burst of air. Any ideas?????

 

no idea...is it very noticeable??

 

Yes, very noticable with the hood up and standing near the truck. I was told that it may be a release valve from the compressor pump. When it reaches 450 psi it releases the pressure. Any feedback???