pshhhhhhhhhhhhht sound

[snmazer]

I own a 2003 H2, and lately, while driving, I hear a "pshhhhhhht" sound, which I think is coming from under the hood on the passenger side. Ususally it lasts for about 2-3 seconds the first time, right after start up, then it will do it a few more times while driving, decreasing in length each time.
Anyone else here experience that or any ideas what may be the problem?
Thanks
Scott