Usually, it's because they're turning too fast. Find out what the tip speed is in miles per hour, km per hour, meters per second, whatever units you can tolerate. Compare the result to mach speed.
Airfoil shape? Blending at the tip? Sharp trailing edge? There could be a lot of other reasons - how can we guess with no information?
Wait a sec: your question does not fit the context implied by the body of your posting. Have you already built blades that turned out to be noisy (as the title implies)? Or do you want to avoid making noisy blades in the future?