When you hear the tapping noise are you able to isolate where it comes from? You might think this a little goofy, but if you take a yard stick and place it against the engine and cup your hand around the other end and put your ear to your hand, it can help you locate where the noise is coming from. It works like a stethescope.
I don't have any idea what the noise might be, but at least when you check with your mechanic, you will be able to help him isolate the problem area a little more quickly and might also save a little bit of money in doing so.
Good luck,
Eric