I would of never believed a cleaning would have fixed the air issue but it sure did. Thanks for the post, I was almost ready to pull them out and sell them. They are quiet again and I know the fix when they start to trap air again.This is the first sign that the impeller and cage need a good scrubbing. It happens about once a month for me. Once I hear the noise and see the bubbles then I remove and clean during my next water change.
Jeff