I have a 1999 F150 4.2 liter V6. I had just changed my oil and about an hour later while I was driving I heard loud knocking noise coming from the engine. It sounded it like it was coming from the top and bottom. I checked to see if it had oil in it. It did. I've taken it to 5 different mechanics for them to tell me different things. The first guy said that I had bad lifters top and bottom. The 2nd guy said my pistons were knocking against the cylinder. the 3rd guy said I needed another oil pump. The 4th guy said he didn't know. 5th guy said I had compression but it sounded like the engine. The truck is not leaking. The headers are fine but it makes this loud knocking sound. I don't know which one to believe or if its any of the above. I think someone is trying to take me for a ride because I don't know much about trucks. Help? What could it be?

 

Is the knocking all the time? Idleing or crusing? Just on startup? Could be any or all of those things that were mentioned. really depends on when it's doing it.

Is this the first oil change you have done on the engine, or have you done it many times before?