Is it doing the same on overrun, i.e. when you blip the throttle and engine revs decrease? I have the same kind of thing on mine, easily reproducible doing the above, and also (oddly enough) putting the engine under load by slowly reversing up a steepish hill (speed up then slow down type thing). Mine's a diesel though, dealership said it was a normal turbo noise. I don't think so.
I'm just trying to get more evidence on it, but life takes over :-/