noise help

[Stroked]

2006 F350 xlt 6.0 automatic
silverline 4" turbo back with a muffler, no cat

under load, at operating temperature, has a noise that was explained to me as sounding like a rusty gate opening, doesnt stay, it is intermittant. 2 ford dealers later, they put new gaskets on the y pipe to the turbo, and did something with the egr, sound is still there after picking it up today. first one like it either dealership has seen, has anyone dealt with this before?