When was the last time you put a new battery in it? Sure sounds like the battery is on it's way out. The clicking is the starter relay opening and closing quickly. The battery is strong enough to close the relay, but then the starter pulls the battery down very quickly to the point that it can't hold the relay closed and the relay opens back up. Then with the starter off line the battery once again has the power to close the relay and the cycle starts over.

Scott