I HAD THIS SAME PROBLEM MY BACK PASSANGER SIDE SPRING WENT, IT COST ME £250 AND IT HAD TO BE REPLACED. IT STARTS OF NOT TO NOTICEABLE BUT IF YOU LEAVE IT (LIKE I DID) IT GET WORSE. THAT PSHHHHHHHHH NOISE IS WHEN ITS PUMPING AIR BACK INTO THE SPRING TO GET IT LEVEL, BUT ITS NOT GETTING IT TO THE CORRECT LEVEL ITS LEAKING. IT WAS A TOTAL NIGHTMARE DRIVING, BOUNCING UP AND DOWN, EVENTLY THE FUSE WENT SO THE AIR NO LONGER PUMPED UP THE SUSPENSION (I didnt realise this at the time and had to drive it to the garage on three wheels! lol!) EVEN WORSE THAN BOUNCING. I WOULD GET IT REPLACED SOONER RATHER THAN LATER