sacrificial finger tips. after a few shaves you may find you're not choking down and the sides of your brush seem to have grown eyes. it also help you'll not mash the bristles like you used to, it wasn't as important as you thought.
I've been using an Edwin Jagger bowl for about 6 months, I stopped using it because of this particular annoying sound. I recently bought a wider scuttle made by Steve Woodhead, it works like a charm. The depth is perfect as the handle doesn't touch anymore the bowl.