The exchange below is relevant to engine noises and separating valvetrain issues from actual piston slap. A footnote to the piston issue is that these later Jeep 4.0L inline six engines have a tendency to break piston skirts. This problem has a two-fold cause: 1) short and narrow skirts on the 4.0L pistons and 2) close piston-to-wall clearance (0.0013" to 0.0015") to compensate for the short piston skirts and meet EPA/CAFE fuel consumption goals. Lighter oil (5W-30 and 10W-30) recommended for these engines compensates for the narrow piston-to-wall clearances and helps achieve EPA/CAFE fuel ratings...There is much more in the exchanges below. Take time to read through the material.