Starter - bang!!!

My kz1000 csr will randomly make a frown-inducing noise when I turn over the engine with the starter SOMETIMES. It will randomly make a terrible metallic clank like a hammer hitting the engine and when this happens the engine will not start usually; either because i freaked out or there is some mechanical problem. Here is my best worded impression:drrr-duh-duh-duh-duh-CLANK! I do not like this noise and it sounds like its from the starter area but i cant be sure. When this happens I try to start the bike again and it usually does. Does anyone know what this could be? FYI this does not happen often.

If after the duh,duh,duh, Clank. Does starter continue to spin or just nothing? More pronounced when engine is cold or hot? 

It's hard to remember but I thought that I could hear the starter a bit after the clank. I only noticed this happen on a cold engine but I do not remove the possibility of it happening on a hot engine.

Could a handful of things. These Mitsuba starters came in a couple versions, 2 or 4 brush. Yours i believe is the 4. So if starter isn't strong enough or supply of amperage lacking you can get that backlash clunk. Another issue could be the 3 Allen Sprague starter clutch bolts are working loose?