thats a real strange one.......

only thing i can think of is if the turbo has been built up and the small paper style gasket has not been fitted behind the copressor housing, (where its bolted and splits apart)this can cause the impellor blades to scrape on the inside of the housing....

but then again it would have been balenced when built i hope... so it would have been heard....

fooked if i know....... good luck tho