YOU can quickly determine if the problem is due to low oil pressure by having the oil pressure measured. A mechanic should also be able to tell if the noise is coming from within the engine and, if so, whether it's coming from the top of the engine or the bottom. If it's a light tapping, it's more likely to be coming from the top of the engine, possibly a tappet. A cracked manifold should be easily found on inspection; same with a valve or something in the exhaust system. If it is in the engine, the only way to determine the cause is to tear it down. Rover has gone from the market, but I would seek out one of the old dealers.