[boxwrench]

Is the tapping at engine speed or slower? This can help determine whether it's the bottom end or piston slap, or valvetrain noise.

If you try to make a recording of the sound, that might give folks here a better sense of what's going on. Try recording from the side vents, under the middle of the block, and towards the sides under each head. That can help point you to the source. Even get farther back under the transmission, too, which can help isolate those weird sounds as well.