I have a 2000 Ford f150 with a 5.4 liter when I start my truck sounds like someone pecking on the manifold or a rock in it I know its not the engine because it sounds like its coming from behind the engine it also does it when I drive in o/d and switch it off it happens or I can be parked with my truck in park rev it does it then too can yall help me out

 

Plugs loose.