gearbox question
 

This weekend I put my bike on my friend's stand to clean and lube my chain. I had been hearing a rattle occasionally while riding. So while it was on the stand I thought I'd start it and let it run to see if maybe it is a cctl issue. While it was running we noticed that when it would make the rattle sound the chain would appear to kinda jump coming off the front sprocket.
Any ideas what might cause that?

too loose chain (or chain has tight spot or stretched spot) or sprockets not properly aligned would be my first guess

You'd need to be more specific with how exactly it's "jumping". First of all, with all the moving parts there, you're gonna hear some noise. Especially if you are next to it with no helmet on (do you ride with a helmet? I have never heard my chain make noise while riding except when I put a stoppie down). If it sounds like it's clacking when going over bumps or changes in accelerating, then it needs to be tightened. If the axle is already very far out, it needs to be replaced.

I guess it's also possible that your front sprocket's teeth have warn down and it's causing the chain to move more than usual.

Really the best diagnostic is to just make sure everything is in spec. Take the chain off, inspect your sprockets, tighten everything down, tighten the chain, lube it up, etc. If you find a problem, great. If not, you know it's the way it should be anyway and you probably just heard a typical bike noise.

A bind in a link well cause this to (A link that is locked in place).
check your chain links and see if they all lay down straight when they are on the chain slide.

I guess I will have to pull that cover and check the front sprocket out