Yeh, I had a very similar problem a few months ago. Was late at night, some bogan in a VT pulled up next to me and wanted a run, so i gave it to him then i realised next set of lights there is this flutter noise coming from the turbo =(. First thought was something wrong with the manifold, but turned out to be liuke yours, the gasket. Bought the gasket the next day and installed it myself with a friends help, was a ****en big job for someone that didn't know what we were really doing