Go to www.pistonslap.com to see it that is what you are hearing.? In my '07, it's a lifter that related to positioning of the cam at the high-lobe on piston 1 or 4 valves at engine shut-down.? After sitting overnight, the oil is bled off creating slop in the valve train on a cold start.? Mine takes a minute or longer to build oil pressure back to within design clearance or required valve-lash.? Pistons 1 and 4 on the driver's side (6 and 7 on the passenger's side) that are controlled by the AFM system that bleed off hydraulic pressure to collapse or de-activate the lifter when it shifts to 4 cylinders operations.? In other words, those 4 cylinder's valves no longer open during a 4 cylinder operation.
Several owners have reported this problem including "yours truly" however none to my knowledge, have not been able to get their dealer to respond to the issue.? While some say it's just an initial cold start noise that is harmless, I say "B.S.".....a tap like I have will eventually damage the affected rocker arm or valve top or both well beyond the design tolerances.
As an experienced engine builder, I have conducted an experiment with manually turning the engine to collapse the lifters and have reproduced the tap each time.? I have retained an independent, certifed mechanic who is following this same procedure.? I expect a written report as evidence in forcing the dealer to make the necessary repairs.? If they balk again....we go to court.