Im certainly not going to compete wtih Nick or Lu on Technical reasons. Too me its been sort of simple. Lots of parts at various speeds. Engine, Transmission, Main Rotor, Tail Rotor and tail Rotor Drive train. Has to be Balance and Resonance problem at various speeds.
I do recall picking up a Cobra from Gulfport many years ago for a long cross country. The Tail Rotor was so out of balance I thought the tail boom was going to break off. It took the Mainteneance folks about four sets of blades and a bunch of weights to get it flyable.....
What is it someone recently said. A Helicopter is a massive amount of Metal Fatigue circling an Oil Slick.