I've had this problem for a few months now on my Mk3 Valver. Almost every time I give the car a good blast when I change at about 7k rpm there is a loud bang from the exhaust. When it first happened I thought it was something hitting the underside of the back box, but it happens every time. It does'nt have any effect on performance or reliability but is just odd

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Anyone got any ideas what it is, perphaps somethings on its way out??
p.s.
By the way the exhaust is completely standard and about a year or so old