Would a smoke test determine if it's a warped manifold?Does it "squeal" when accelerating in 1st or 2nd?
Does it doe it more when the engine is cold?
If so, you have a warped manifold. Common problem.
If you are "out-of-warranty" talk to Jerry at Tofosis Performance (Has a forum in the Sponsor Forum). They make ported, ceramic coated manifolds that should reduce this problem (caused by too few bolts by Ford).
Did they check it when the engine was cold, or when it was heat soaked, and was it making the noise while they were checking it. To me, it really sounds like an exhaust leak up front.The exhaust manifold had been the first thing they checked. Smoke tested it and said it was good. Just took it back today so I'll let everyone know if they find it.