Couple of questions. Can you isolate the "shhhhh" sound to anywhere? (engine compartment or under the dash) Is this sound related to outside air TEMP? (gone when outside air is 40 and above). If the car has 15K miles on it, it is under factory warrantee until 36K (or longer if you have an extended Warrantee). I would let for "Ford garage" deal with it. If you can isolate the physical place where it occurs it should help the repair facility. I would think it is either a mechanical noise of a vacuum hose (or fitting) problem. I know I am not much help to assist you, but it is rather difficult to try to troubleshoot this over the internet. Keep us posted on what you find out and good luck.