I'd pull the tail gear box and check your gears. Some people have problems with wearing out the gears. If yours are close to worn out, they might be skipping a tooth causing the clicking.If thats the case you aren't far from possibly losing the tail altogether. I pull mine apart every few gallons just to check it out and regrease it. I've not had any appreciable wear but others haven't been so lucky.
Later,,,basmntdweller