You say you hear a clunck or 'knock' before it cranks....have you pulled the starter to insure the bendix isn't buggered up and the sound your hearing is it engaging in the flywheel starter ring? I have also seen starters that would hang up when starting, if the teeth were screwed up. When that happened, instead of kicking out like they were supposed to, they would motor around (((until something blew up or the engine was stopped and they released until the next time,)) and would cause all sorts of odd noises.
On the opposite end of the spectrum. I had one in a CAT scraper years ago that made no noise at all until you wound the engine up to a high RPM, and then it sounded like some sort of jet turbine. When pulled the starter was so hot I had to use welding gloves just to get it on the ground as it was too hot to even think about handling otherwise.
Good luck and let us know what you find.