2005 pontiac g6 gt with v6 engine

my daughter is driving a 2005 g6 gt with v6 engine.. it sat for a bout a week, she return start and now theres a knocking thats steady at idle somewhat speed up under acceleration and intermittent drop in rpms at idle (stopping at stop lights) .. i hear this is a common problem but i can not get a straight answer as to what is the issue and how to correct it. I checked oil levels and i put dry gas in the tank to rid any bad gas water mixture issues..still nothing.??? im out of ideas :S
it did throw a cei code p0420 for catalytic converter efficiency ..not sure if thats related to this issue