1997 Ford F-150 Oil pump, engine knocking

My, '97 F150 w/about 89,000 miles started making a noise sometime last week, I took it to the shop and they had to tighten a cover on the front of the catalytic converter, but then they noticed it was knocking and said the engine would probably need replaced they put some thicker additive in there to see if it would help but it doesn't seem to be helping, but then my neighbor who is pretty good with cars said it might be the oil pump, it sounds like it's not getting enough oil through it. That makes sense because the oil pressure gauge has been acting funny too, it moves up and down for no reason and lately the pressure has dropped, I'm not sure what to do? Can somebody give a good answer.