FM synthesis is the process of changing the frequency of one oscillator based on some other oscillator. Doing so creates “sidebands” which are extra frequencies above and below the original oscillator’s frequency.
At a relatively slow rate, this produces a vibrato effect when the modulation is performed by a sine or triangle wave. When the modulating oscillator is a square wave, the effect is a trill (rapid alternation between two notes).
Speeding up the modulation into the audible range is where we find extra frequency bands known as sidebands. Unless the two original oscillators are harmonically related, the sidebands seem to be random (although they are actually mathematically predictable.)
With so much harmonically-unrelated content, the resulting tone can sound metallic—similar to ringing a large bell. You’re left to shape the harmonics with filtering or by adding more oscillators to reduce the apparent presence of the harmonics.