I know this is an old thread, but thought I would bump it instead of making a new one. Could this not be resonance from the worsening shaft play?
This happened on a stock R33 I had years ago, and it never got worse, but the car was rear ended so I never got to see how it turned out. Now it's happening on my new R33, which had never made the noise before. I've had exhaust leaks before and it doesn't sound like that, and if it were an exhaust leak wouldn't it stick around under load?
This sound lasts for about a second, and it's during transient throttle, like the turbo was spooling a bit but is now slowing down. It seems to me like the turbine is spinning up, under load so it doesn't wiggle, then when you're off throttle, it's briefly unloaded as it's between vacuum and boost, so it can wiggle a bit and begin to resonate.