Sounds like you have a chain and/or sprocket problem. A quick way to check for chain and sprocket wear is:
- Lift the chain with your left foot, taking all the slack out
- pull the chain away from the rear sprocket, in line with the end of the swingarm
- if you can pull more that 1/3 of the height of the sprocket tooth, the chain and sprockets are worn out and need to be replaced.
- if you don't feel any noticable movement, your chain and sprockets are in good condition.
Also, look at the sprocket teeth. If the shape of one side of the tooth is different than the shape of the other side, the sprocket is worn out.
Clicking, clunking, slapping, etc. are all signs of a worn drive chain.
Make sure you replace both sprockets when you replace the chain. Worn sprockets will eat a new chain in only a couple of thousand Kms.