[barron64]

You can rock the pistons in the bore of any engine if you try. They are tapered with the smallest diameter being at the top which makes them easy to rock. They have this shape because the top of the piston expands due to the high temps of combustion. This is why they need this extra clearance. The whole deal sounds fishy to me though. If it only made the sound when you put it in reverse it is not your engine making the noise.