Is the noise consistent in tone and speed, or erratic? Is it worse/better if you let off the gas? (Additional oil seeming to help aside) it might be spark knock, which sounds like shaking small rocks in a metal bucket. Knock would be worse in high load situations when you are in the gas and would stop when you let off.The noise the engine makes kind of sound like (past times in my life with different cars) a low oil engine noise in its frequency - how often it ticks. It stays with RPMs, so it's fast.
But I didn't want to say it sounded like "tapping" because all the tapping I've ever hear had a high pitch to it. Mine has a lower tone than that - almost like if you hit a small metal bucket.
I hope that helps a little?