I think I would start by disconnecting neg bat lead from post. Then see if noise is gone. If gone electrical. Have to start searching for that. If nor mechanical and you need a helper to help locate noise.
Pay close attention to 'o' ring gasket on back of oil pump. Darned things seem to be able to slip out of place and lets the oil pump suck air. Makes awful commotion as tappets clatter worse then Mazda v6.