pshhhhhhhhhhhhht sound

[snmazer]

It does only seam to happen when the engine is "cold". It is about 60 degrees here, but I didn't drive my Hummer for about 20 hours. It made the pshhhhhhhhhhhhht sound about 4-5 times. Immediately after start up, then tapered off as I drove. Ther were about 2-5 minute intervals in between the annoying sound. It seems like it quits after the engine is warm.
If it were an exhauste leak would it be a continuous sound though?
As far as the AC, I haven't used it at all since last summer, not sure if that matters or not.
Thank you so far for the replys, and if anyone else has any ideas please post them.
Scott