By "ratcheting" do you mean the chain feels like it's slipping causing a jerking motion while pedalling or do you mean a clicking sensation while pedalling?
If the former, that's usually caused by an improperly adjusted rear derailler. The Park Tool website has some good information on how to properly set up a rear derailler. It consists of setting the high and low limit screws then adjusting tension to make sure that the indexing properly aligns the rear derailler with each cog.
If the latter, it could be caused by the upper rear derailler pulley teeth hitting the cassette teeth. This should be able to be dialed out by adjusting the b tension screw which is right near the rear derailler mounting point on the frame. Screw it in to move the pulley further from the cassette.
I'm sure there are other causes but that's all that's coming to mind for me right now.