When I was interning at Dodge dealership we had a few cars and vans comming in doing the EXACT same thing. Start it up, does alright a little knocking, drive down out of the dealership and then it knocks more but not real loud at all and then the oil pressure drops when you apply the throttle more. We took a look at the engines took the heads and all off and oil build up in a few that looked like a few barely ever had an appropriate oil change and we had to just go put in a WHOLE new engine and then the others needed bearings and all or rebuilt. Take the engine apart and see what's wrong with it.
You won't need a whole new engine or nothing of course we only did that because one was a Altel van with the tires all in wire from worn so much and not taken care of so the company brought one over. And I know I am not the one here that gives the info or nothing and I get crap about how I should not say nothing but unless someone dissagrees with me here then I know that could be a possible problem.