I have heard of one engine that had new barrels and pistons under warranty for "piston slap" but that is the only one, so not a common problem. As you have just had the Bleazey Centa fitted (I assume the problem was not there before?) that could well be the source of the noise, as Paul V suggests. The Centa spline adaptor requires careful fitting, something complicated by the S&S crank splines not being made to very close tolerances it seems.
You can confirm any play in the Centa by removing the camchest cover and putting a socket on the end of the crank pinion retaining bolt and an Allen key on one of the screws on the outer edge of the Centa case. Hold the crank still with the socket and wiggle the Centa housing with the Allen key, any play should be apparent. If you have any play, that is the problem and the engine needs to come out again to fix it.
