Strange Hissing Noise

I have a 2002 CXL with ~30,000 miles. Today I noticed that there is a quiet but noticeable hissing noise coming from somewhere in the driver's side back wheel well. I thought it was the tire, but I think it's a line of some sort behind the tire. It isn't leaking any fluids. Suggestions?

hissing would suggest a vaccume line. Jack it up take the wheel off, put the car on a jack stand and start it up. Should be able to pinpoint the leak that way.

Do you have tow package? If so I would expect a leak in load leveling system