Does the noise go away when the oil pressure light goes out? If so then its your rod bearings.

If it doesnt go away until the engine really warms up then its b/c your cylinders are tapered and you're getting pistion slap.

You should be using a heavier oil with that much mileage.

No matter what problem it is you are going to need to rebuild the engine. So I would get some oversized pistons and rings and get it bored out and get new bearings, ect.