01 DGC Hissing noise under dash

[CH41]

I bought a 01 DGC Sport with 78,000 miles for my wife in april of 09. We have about 93,000 on it now. It has the 3.3 engine. My wife is complaining of a intermitant hissing noise coming from under the glovebox. I went out and listened for the noise and heard nothing. So I turned on the heat, then changed to AC and to different selections. I could not replicate this noise. I took the van for a ride a few days later, and I was just about to give up on this so called noise. Then low and behold the hissing under the dash began. It was almost similar to the vacuum hissing sometimes associated with changing the selector switch from floor to defrost etc. It is somewhat louder than that though, and changing heat, AC and selections makes no change to the noise. everything still works, AC blows cold air and heat is hot. Anyone else ever have anything like this? Or does anyone have any direction as to where I should start to repair this? Thanks in advance!

[RIP]

The climate system uses electrically actuated actuators to operate. No vacuum lines to leak. One reported source of hissing noise is low refrigerant in the air conditioning system.

[CH41]

Thank you for the info! I was thinking the same thing about the refrigerant being low. The only thing that made me unsure is the fact that the AC still blows cold air. I realize now with the cold temps of winter here in pennsylvania it would be tough to tell without a gauge. But we have had this problem since august, and still had some 70-80 degree days. The AC system seemed to function flawlessly. And now with the heat on in the cold temps, the AC is turned off and the intermitant hissing continues.