On the ducted tail rotor on the Eurocopter product line, the blades aren’t axisymmetrically arranged- there is an unequal amount of space between some of the blades- it’s balanced, but I was told by one of the company pilots that this was done to reduce cavitation and thereby reduce noise.

If you look at the tail rotor of the Apache you will see that the four rotor blades do not form a plus, but an X, where the gap between opposite sets of blades is equal but if you go around the blades the first gap is different from the next all the way around. A similar design is used on the Mi-28 helo. Its purpose is to reduce noise. (Tail rotors are notoriously noisy and are often second only to the engines… though except in helos with too few rotors like the two blade Huey.)