Exhaust leak, but everything is tight

i have a LOUD exhaust leak that sounds like a pissed off rattle snake. only hear it under boost. (its not knock) ive just finished checking all the bolts on the header, UP and turbo.

what else could it be?? i havnt had anything apart recently. could the wastegate make that much noise? how can i check? i have a Perrin IC, i dont think that O-ring could just start leaking could it?

i have a Prodrive UP with the flex joint. it looks in good shape, but maybe something inside the joint is rotted and leaking??

need some ideas