Slats
While many focus on the blades for freq problems, the damned engine is
turning faster than any other components and can create confusing
vibrations for the unwary. Of course if you are flying a turbine, we
have a different set of conditions to deal with.
I daresay, helicopters pilots have the most finely tuned sense of
vibrations of most trades, except perhaps musicians? And its amazing to
me how many pilots are musicians!
Cheers