Why does my ice maker make a knocking sound and not produce ice?
Our ice maker recently had its water inlet freeze up, preventing water from filling the trays.  The mechanism kept dutifully trying to push nonexistent ice cubes into a bin that never filled up, which might account for the knocking noise in your case.

To defrost it we just unplugged the unit, took all the food out of the freezer section, and left the door open for a couple of hours.  It wasn't long enough for any of the frozen food to defrost, and since we were careful to minimize opening the refrigerator section's door, the food in that part stayed cold enough to be safe as well.