with the wastegate closed feel around all the flanges/joins and see if you can feel any hot air being puffed out. you may be able to see some black soot around the flanges or even manifold runners from a crack where a leak will be present.
it does sound a little to me like a leak. when the wastegate is closed it may be allowing just that right amount of back pressure pre turbo to make the leak audible. when you rev the engine can you hear it more? specialy when it's cold.
small manifold leaks are sometimes a PIA to locate, they can radiate a ticking sound off curbs etc. and drive you mad!
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