What are the pellets made of? Any form of calcium on the pellets reacts with water and produces air bubbles. If there is not much calcium in the pellets then the air bubbles will be small and cling to the pellets until the are knocked off by vibration or water movement. A kind of slower version of Alka Seltzer.
I can't believe I am typing this. Way past my bedtime and I am typing a load of rot.