Sorry, can't remember the diameter. But go back and tighten all of the clamps on the pressurized components and go out and drive it. I do know that I came across a post, can't remember if it was here or elsewhere, but an individual stated that he had a similar noise. Turned out that it was a leak at the
exhaust manifold where it meets the head. I guess the gasses leaking past the gasket and the gasket acted as a reed. Good luck and let us know what you find.