So my truck ran fine on Friday, left for the new years and went to start it up today. Starts fine, idles fine, revs fine. But put in gear (forward or reverse) there is a loud knocking sound. Put it in park Rev it up, and not a single knock sound. What could this be? It is definitely coming from the engine bay.

 

torque converter maybe?

 

Could be a cracked flexplate. They commonly sound like a rod knock. Sadly, you probably can't check without pulling the motor or trans... Worth trying to look up in there though.