Start the engine
from cold, rev it up to the level where the tapping is consistent and go down the line, touching the valve cover above each cylinder. If it's a loose rocker (which it sounds a lot like it is), you'll know it when you feel it. If you run that engine
at the level you are running it now to get to the 'rattle' with the valve cover off, oil
is going to be everywhere
Stethoscopes work well, as do screwdrivers and broomsticks, but with something that pronounced, you probably don't even need one... Is the sound obviously from one side of the engine or the other?
I've always had the best luck adjusting hydraulic lifters (valves) cold and static, as described by Crane Cams here:
It's easier to do than to read about doing, will only take an hour or so, and at worse would eliminate one unknown. At best, of course, it would solve the problem.