Most all "piston slap" complaints on Northstars end up being related to excessive carbon build up in the combustion chambers. The carbon in the squish areas of the chamber builds up and contacts the piston at TDC causing it to rock slightly (harmless) and make the piston slap noise. This is almost always more prominent when the engine is cold due to the increased clearances until the engine warms up. It is very easily prevented/fixed by a few good WOT blasts on the freeway to clean the carbon out. Much easier than any other possible fix. Just an occasionaly dose of WOT will keep the carbon and piston slap away.