its fairly loud, can hear it outside quite loudly.
performance doesnt seem to be affected
the sound you are describing when on boost sounds similar to my problem last week, was only getting noise on boost, not on idle so ramps were pointless
are you losing pressure just after peak boost?
mine was and it turned out to be turbo to manifold gasket, didnt chatter though, just made the noise you described on boost
just a thought .........