Hey all, my GS500 started making a rattle in the engine the other day. I pulled the clutch side engine cover and found (the item in the picture) is loose and I suspect that's the culprit.
I tried just simply tightening it but it must be connected to the cam chain or something and the engine spins when I try to tighten it.

What exactly is this and how do I fix it (tighten it)?

That's the balancer shaft and it's geared directly to the crasnkshaft so the crankshaft turns in the oposite direction when you turn the balancer.  If you can hold the big nut on the crank you can tightenen the one on the balancer shaft but I suspect it's more likely the balancer shaft bearing that is worn and knocking.  Can you move the shaft up and down like Addy does in this vid, if so you have a bigger problem than a loose bolt:

http://www.youtube.com/watch?v=Atq4nKfh7kg

.. the crankshaft connected to the balancerthing,  the balancerthing connected to the bearing, the bearing it has a wobble?!  ....... uh oh!

(Ala Dr Nick from The Simpsons)

yea it has up and down play, and maybe 1 mm of in and out play...Guess I need to look for another engine.

I remember reading somewhere of someone using a bandit 600 engine in a GS, any info on this?
Or any good places to get a used engine?

That is awful to receive such news. I recently hear an unusual knock from the lower part of my engine. It is only at low rpd, but not constant, only when I accelerate. It does it randomly, it does not depend if the engine is hot or not. I need to open it to check if I can see anything inside.