The propellers coming with the HoverGames kit (the same as provided in the links above) can sometimes have balancing issues: one blade can be heavier than the other, which could cause unwanted vibrations. This inbalance can be removed by adding or removing weight from one of the propeller blades (sticking tape underneath the blade or by sanding it down).
To check the balance of a propeller, you can use special propeller balancing tools, which can show you whether one of the propeller blades is heavier than the other. Some people have used very basic setups, such as an iron nail. Others have used dedicated propeller balancer shafts such as the one linked below.
