Engine rattle

I bought a non-running '78 KZ650SR about a year ago and finally got it going a couple days ago. After letting it run for a few minutes I started fiddling with the choke and noticed a rattling coming from the rear of the crank case, near the sprocket when she idled down. Kinda at a loss for how to go about fixing it.
(Have a video of the rattle, but its to big to upload to this site.....)

They all sound rough from cold, piston slap as the ally pistons expand, clutch basket rattle as the internal shock absorber rubber dampers age, camchain and primary chain rattle... When it is hot, and you stick a rag over the end of the pipe, if i whirrs like an electric motor off idle then you are probably ok.

Balancing carbs will reduce this cold idle racket.

+1 Probably primary chain. Usually this can be reduced by syncing the carbs. Also, keep the idle around 1000-1050 rpm rather than too low. Ed