I'm getting tired of debugging this noise, which seems to come from the back. I have sent it to the bike shop, for the second time, and they couldn't fix it. This sound came around 2 weeks after I bought it.It's not just sound; you can feel it in with every pedals revolution.
They guy said the bottom bracket was loose, and that he greased, lubed and tightened everything. For a while it was smooth, but 2 hours later it returned. It seems to come from the back (I always believed it was the pedals because you felt it there), but once I went down and got a friend to ride it it most definitely comes from the gears in the back. It can't be replicated but under tension and the most use the worse it get. I only use the bike to go to work, I honestly don't know how it got that bad.
Update: they replaced the casette and I haven't heard the sound so far.