I don't know about the hissing but it sounds like the air ride pump may have gone out. Mine went TU last year and once all the air went out it rode like a 5 ton truck. When mine went out I did get a warning on the dash. If it was the struts the ride should not have gone to hell all of a sudden. I'm a newbie here but I'm sure there is a thread for the air ride somewhere here and lots of helpful folks. Good luck.