Lela has always howled like a little wolf, but her full blown yodel is pretty recent ( she is 2,5 y. now). Usually when she feels left alone or doesn't see us.
Binti (3,5 y.) howls a bit too, but less than Lela - she screams much more (at cats). She also has a deep humming sound, not unlike a small outboard engine that is not quite running properly. We hear that a lot when preparing her meals.
We hear from other owners that every individual has a different repertoire.