Hissing sound from the rear after parking the vehicle on 2004 Lexus GX470

I have a 2004 Lexus GX470. After the vehicle comes to a complete stop and I put the vehicle in park and get down, I can hear a "hissing" sound from the rear of the vehicle. I've noticed this doesn't happen all the time. Do you know what might be causing this? Sounds like air pressure release of some sort...not sure what it is. BTW, I've only had the vehicle for a month.

Asked by for the 2004 Lexus GX470
Not to be rude, but, to the two grown adult males who said they heard a "hissing sound" coming from theirr $53,000+ SUV's and couldn't figure it out. Your an embarrassment and should have to drive a Geo Metro for punishment.
The embarrassment is misspelling "their" and "you're."
2 more answers
I have a 2004 GX. The hissing sound that you hear is the self leveling suspension. When you get out of the truck and you reduce the weight on the suspension the truck raises up a little. The hissing that you hear is the bleeding off of air from the rear air bags to level out the suspension.
I have a 2006 Lexus GX470 and exact same thing. Did you ever find out what it was?