if in fact the engine was running with little or no oil pressure, you could have wiped out the rod or main bearings. This would usually cause a knocking sound when the engine is running. Another possibility could be the pistons themselves. Without proper oil pressure, they can score easily, which in turn can cause a piston slap noise. I wish you luck with your problem, but if either of these conditions exist, there is no easy or cheap fix.....Good luck