sounds like the "bearing bushing" in the starter motor. They are made from a soft yellow metal which can get contaminated from dirt and dust which makes them wear just enough to let the armature spindle oscillate at a certain speed and that's what you hear. The bearing bushes are made from "self lube" metal, means there's no oil or grease used.
Cleaning the starter out to get rid of the dust etc might stop it but it's a real pain to rebuild the motor again because of the spring loaded brushes that run on the commutator. I did mine once and it was a pain.
I've added a pic of a typical starter... not a Ducati. You can see the "bearing bushing".
Old cars use to do the same noise.