What's this part on my water main and could it be making a consistent tapping noise?
That is nothing.  It's just a tee fitting with a short piece of pipe that is capped off.  That shouldn't make any sounds or cause any issue.  As for why it is there - no telling.  Installer could have ran out of couplings and had a tee handy, or read a diagram wrong and thought there needed to be a tap there.