Chain Noise I identified the problem, don't know how to solve it (chain/cassette?) I bought a bike, second hand, yesterday. The bike looks quite new and it is ok, but I notice that with some gear I got a clicking noise. After inspecting the bike, I notice that the chain does not flow steadely on the rear cassette, and this make the clicking noise. 
I don't know how to be more precise, so I uplodaded a video where the problem is quite clear:


                
             (second 5/6)

It seems to me that the chain is really close to the other disk (is it called cassette?), maybe because it is misaligned. 
Of course this is not only about the noise, I am afraid that it will ruin the chain/bike.
Plus, if this is a serious problem, maybe I could just ask the guy I bought the bike from to take it back. However, I would be totally happy to solve the problem myself.

Please consider that I am a TOTAL beginner :)
Thank you!
You may want to read up on how to tune a rear derailluer, but if you just want to get going, all that is needed is a minor adjustment using the barrel screw (clearly seen in the video) where the cable comes in. Turn the screw clockwise 1/4 turn at a time, and the chain will move away from the cog (The cassette is the full set of cogs). Trick is not to make big adjustments - no more than 1/4 turn at a time, and count the turns so you can go back to where you started. 

Once this is just clearing and not catching, check the gears are all working properly. Sometimes, especially with cheaper bikes and old worn out gears, its not possible to get it perfect, and you need to compromise a bit.