Rough idle 2000 Ford Taurus

I am noticing a rough idle where the enging will race to 1500rpms from start up and then jump down to 700-800 and it will be stumbling really rough. I found codes P407 P401 and have since changed the EGR, DPFE sensor, cleaned the MAF, IAC, changed ther plugs, cleaned the throttle plate, changed the PCV, so I am thinking it may be a vacuum leak but where? I don't see and cracked hoses. When the car idles high the engine runs smooth, once it starts idling 700-800 rpms and stumbling I hear the hissing sound around the EGR area that I never recall hearing before. The funny thing is the hisdsing sound disappears when the car is reving at higher RPMs.