Holden Commodore SS clattering on start-up

One morning at start-up, my Commodore SS made a terrible clatter. The oil pressure was fine, but I switched it off to be safe. The dealer told me if it goes away when it warms up bring it in. Starting the car again the engine laboured to crank over, but started, still clattering until it warmed up. A sample of oil showed there were fragments of metal the size of a pinhead in it. The dealer said they could not hear anything wrong with it and refused to investigate it further. The car is two years old with one year of the warranty remaining. What can I do?