Yours is the same as mine then, Nick!
I noticed it in daylight recently whilst, erm, "playing" with a colleague's tuned up MR2 Turbo behind me. I'd seen it regularly in the dark with headlights behind but mainly at the initial opening of throttle.
Now I notice it quite a bit but mainly under heavy boost and sort of mid to upper rev range.
Not sure if it IS boost hoeses though as mpg is very good now it's warming up a bit - getting closer to a real 50mpg from a tank!
One thing I haven't tried yet (no willing assistant to hand) is putting a can of 10k boost through it, as the inlet manifold is surely quite sooty now, which will of course restrict airflow a bit. I have the can, just not enough pairs of hands!
edit: should add that mine's on the original MAF and MAP, MAP recently cleaned, MAF never touched.