Not certain of piston skirt failure being the noise although I have seen it only once in a straight 6 Ford. It would only make a brief knocking noise upon accelleration from a stop light...

If your interested. I am going to need to replace the pistons in my stroker block. The ones in there now are .030" over coated speed pro's I think. Not sure if you can use them but, If interested I'll get the numbers. If you can use'em, I'll cut you a sweet deal on'em..