First of all, revving the engine on a stand, will in no way, simulate running down the road. There is no load on the engine. That engine in stock form is only good for around 4500 rpm's. What kind of oil pressure does it have? It could be a lot of different things causing the noise. Lifter, valve spring, piston noise, rod bearing, pushrod hitting the side cover, etc etc. While the engine is out I would think about pulling the pan and checking bearings
