Exhaust Leak?

I've had this very annoying for quite awhile and have taken to a few mechanics who can't find it either. It almost sounds like a turbo spooling, however i don't have turbo. Its pretty loud too. I have replaced the pipe from the manifold to the cat, b/c of another leak. Thats when i started to here this one. All new gaskets were used in the replacment of other pipe. I don't have a cat-back system its all stock except for the muffler. ANy help will keep my hair from leaving my head.