Hi I'm new here and I'm having engine problems any help is greatly appreciated. My 2002 Ford F-150 4.2 Litre is making a loud tapping noise under acceleration (might even cosider it a banging) and while idleing in park or drive. It is in rythm with the engine when it is happening and it fluctuates with the rpm's. I'm pretty sure that it isn't the cats or exhaust but not 100 % sure. I think it could be many things like a valve sticking, bent pushrod, or piston slap. It seems to get better when the engine warms up but not all the time. The engine also idles slightly rougher when the noise is occuring. it is very sparatic and doesn't happen all the time. Any ideas what it could be?

Thanks Matt