Rotor aero engineers have a good understanding of what the sources of rotor noise are. The difficult part is coming up with a low noise solution that does not compromise other areas of rotor performance too much. The digital design and analysis tools available now are quite good. And better methods for visualizing flows during testing, like this Schlieren technique, will make it easier to validate the modeling results.
In addition to improved aero design of blade tips, another very effective approach being considered for noise reduction is variable speed main rotor drivetrains.
Here's a fairly recent paper from NASA on reducing noise from rotor tips.