"fluttering" sound from engine

i have a 2005 g6 and everytime you "get on it" very hard a weird fluttering sound comes from the engine. it doesnt seem to be causing any harm, its just annoying! i have had a couple of other people look at it and none of them can seem to figure it out either. its not the belts and it seems to from like the center of the engine. HELP!