Unusual airframe vibration isn't the most beneficial to longevity especially with the lives of these aircraft.
It is no more harmful than the vibration of a drumhead which makes a lot more noise.
Everything mechanical vibrates some of the time at some frequency and amplitude. When you get a big enough surface vibrating at the right frequency it makes noise. Sometimes its only a hum or whine while in other situations it sounds like a rumble. Most of the time it is outside of both your hearing range and yout ability to feel.