Have a look at the auxilliary belt and the pulleys on it, whilst someone else revs it. Specifically the alternator pulley, tensioner, or the belt itself.
Look for excessive movement/bouncing of the tensioner.
It's a start anyway.
EDIT: I had a tappet noise on my BKD, ran some Wynn's Engine flush through it (purple can) and did an immediate oil change. The noise never came back.