The new batch of PCBs arrived a week ago and I've started soldering all the components. This time the goal is to put up 50 kits for sale on the site, which means a lot of work but also lots of fun.
I updated the board to v.1.1 but there arent any big changes. The major change is nothing more than a bigger capacitor of 200uF instead of 100. It seemed that the smaller cap could give an unstable current or voltage spikes from the stepper drivers, that resonated in the stepper motors sometimes giving of an audible noise.
Also the text is a bit different, with the new website address and also the copper tracks for the power is wider to make sure it can handle all the power, but maybe this also had something to do with the strange whining sound. All in all, they work perfectly now.