Rear noise while vehicle parked

I have a 1997 Transport. I am getting a noise on the rear passenger side while the vehicle is parked, and off. It continues constantly, and is fairly loud. It reminds me of the hissing sound I get when I plug in the block heater. Any idea what is causing this?

Check for the fuel tank cap to be closed tight(not sure on what side it is on a '97 though). The fuel tank is maintained under a slight vacuum. ALso vacuum hoses that lead to the tank.
DO you have an autmatic load leveller? (air assisted shocks can leak after a while..and their air hoses too)
I guess a slow tire leak would have revealed itself....
Please keep us posted when you find..specially if it is something other than these suggestions.

The compressor for the auto-level rear air shocks is in the right rear corner of the passenger compartment, so I think LMP is onto something when he mentioned the air ride.