My truck runs fine and idles good but when I give it gas sometimes it starts knocking (not all the time just at certain rpms) and it seems like it hardly knocks at all when driving

It's a 98 F150 4.6L 4x4

Here's a video I recorded I'm hoping somebody could help me identify this...I'm thinking either rod bearing, lifter or maybe a cracked flexplate it sounds like its loudest at the top rear of the engine but it's hard to tell. I tried using a screwdriver and even a short broom handle and I couldn't pinpoint it either...

http://www.youtube.com/watch?v=68G10LFJ-TI

 

I can't stop laughing at the string. Creative. Could be a number of things. Sounds a bit like an exhaust manifold leak at some points but at others it doesn't. Also sounds like what my old 5.4 sounded like, but not at the same rpms. Could be timing chain tensioners, rod or main bearing, wrist pin etc.. Good video but still a bit hard to pin point without being there and pin pointing it to a particular area with a mechanics stethoscope. Mine ended up being 2 things, tensioners and a rod bearing. From what I heard, when the tensioners go, they eat up the guides allowing little plastic shavings to drop down into the pan and clog the pick up screen.. the problems get worse from there. But again, not sure since yours is a bit different than what mine was. A bit quicker and more pronounced.

 

It sounds like first order meaning it matchs the engine crank RPM. A second order would be half the speed of the RPM meaning more than likely valvetrain since the valvetrain rotates at half the crankspeed. Could be a wrist pin in which case it will probably last the life of the vehicle. They can make noise but rarely become a failure. The modulars are noted for transmitting noise from one end of the engine to the other so the suggestion of a mechanics stehescope is something you should consider. I was thinking exhaust leak as well but it's important to get an exact location or you'll just be chasing a ghost. Sometimes a rod bearing can be determined if you disconnect the fuel injector on each cyl one at a time. When the noise stops at the cyl with the injector disconnected thats usually the bad rod bearing. Good luck.

 

Awesome I was able to clear up the noise when I unplugged one of the injectors is that definitely a rod bearing or could it also be a bad wrist pin?