My Arctica Refrigerator had a leak from the water supply cables. These were changed. While the water was off (we ordered the part) a knocking/tapping sound started. It comes as fast taps 7-8 in a row and some times stops for a few hours, then starts intermittently. The sound comes from the lower right rear of the unit.
I just changed the water filter and ran lots of water to see if that helped. It did not!

There are no other strange sounds... and everything seems to be fine temp wise. ANY clue what it could be? The fridge is 10 years old.

Sounds like the ice maker. Try raising the shut off bar and see if problem goes away.