Hey guys this may sound weird but I had this same noise and I just recently swapped to a lethal h pipe and a gt500 Pcv valve both at the same time and have not heard the noise since. I have a feeling it has something to do with the Pcv valve being more restrictave and not allowing so much oil to push through the intake.
call me nuts timmy, but i dont think thats it. (the car made the sound prior to the GT500 PCV, and my full exhaust.)
Ford thinks its a bad VCT soleniod, ill find out today when i pick her up.