2cm is rather tight. It's the lower limit of the chain spec. I used to set mine at 1"(2.54cm) or a bit looser. That sounds like your chain. Loosen the chain to a full 1" and see if the noise goes away. If you had the chain slack less than 2cm, it was way too tight. Not good for the countershaft or sprockets. If it's not the chain slack, then are you sure the axle is aligned side to side equally? Did you adjust the slack at the tightest spot along the chain run?
Also, how do you have the wheel off the ground when you're adjusting? You want to make sure you've got it up by the swingarm so there's weight on it (as opposed to being supported by the footpegs or something). We made the mistake of doing that once on my friend's bike, so while it looked to be in spec while suspended, when we put it back on the ground, the chain ended up being WAY too tight, and sounded just like yours.