Lots of things. We can start making a list, see how many things we can think of? I'll start with
1) Unbalanced rotors
2) Bent shafts
3) Bearings with defects
I assume you mean jet engines/gas generator related vibrations? If you add turboprops, engine accessories and so on, there's a sh*tload of other potential sources. If you throw recips into the mix.... well, they're vibrating rather severely to begin with but you can still detect lots of things about to go wrong by monitoring vibrations in the right part of the spectrum.
I thought I was doing good trying to avoid those airport hotels... and look at me now.