Hissing Sound?

I have a hissing sound (similar to tire quickly losing air) coming from the left rear wheel well area on my 2002 Grand Cherokee. It started doing this several months ago but I'm not exactly sure when as I didn't notice it till someone told me about it. I can't hear it inside and doesn't appear to be having any negative impacts on performance. Any thoughts? Thanks.

Does it happen all the time, or just when the vehicle is running? Have you gotten underneath to try to locate it more precisely?

If while running, my first thought is exhaust leak or catalytic converter restriction.