I Had one of my 4.2s cleaned as well as a full engine flush and clean prior to an oil change and it started rattling a week later. striped the engine down to find that all the burnt oil that was inside the casing had come away during the flush and resulted in exactly that! the tolerance must have been out due to natural engine wear and the carbon had kept it all quiet and smooth.. removing the carbon resulted in me having a new engine. although the fuel system was clean as a whistle when the heads came off, i recommend cataclean and injector cleaner. but don't flush your engine with engine flush, simply change the oil more often, i do mine every 8,000 miles may sound daft but as soon as my oil goes from a golden honey color to any shade of black its out and fresh in and new filter
