I'm going to assume that you can tell the difference between a knock and an exhaust leak and I have to say also that oil pressure is more likely to go down as the engine heats up because (non-synthetic) oil will become thinner as it becomes warmer which generally leads to lower oil pressure. An engine knock that occurs on a cold engine and fades after warm-up is usually caused by wear on the cylinder walls and is most commonly called "piston slap". Some manufacturers have specified an acceptable amount of time before fade however, this is one of those things that does not get better and indicates possible engine failure on the horizon. If what you are actually hearing is more like a tick or a tap, a faulty lifter may be the problem or possibly even the exhaust leak one of the others mentioned (can you smell exhaust?). This would be located at the exhaust manifold to cylinder head gasket on a single cylinder. A good tool for finding a knock that can be found at any auto parts store is a stethoscope with a thin metal rod on the end of the tube. If you cannot hear any knocking through the earpiece, start looking for that exhaust leak.