Flying things vibrate.
Folks that fly twin engine airplanes know the drill...
Every now and then you have to tweak your power settings to keep the engines singing from the same page.
But helicopters are absolutely the worst!
Engine(s), main rotor(s), transmission(s), drive shaft(s), tail rotor... all vibrate at their own frequency and all need to be harmonizing.
Things that vibrate affect other things, sometimes detrimentally.
I found this video interesting and topical: