Anti-squeal shims correctly fitted and on the correct side of the vehicle (check for arrows or indicators showing direction of rotation, they should point to the front of the vehicle). (mechanics.stackexchange.com)
For example, see this PA - 42, where the mirrored propellers indicate a different direction of rotation for both motors. (mechanics.stackexchange.com)
This image shows Von Karman vortices, a row of eddies that alternate in their direction of rotation. (discovermagazine.com)