I would suggest checking your return plumbing. I had a similar issue where the cause was the check valve.
I disassembled to clean it. When I reinstalled it, I didn't tighten the collar all the way. This allowed air to be drawn into the return line and created micro bubbles.
I disassembled to clean it. When I reinstalled it, I didn't tighten the collar all the way. This allowed air to be drawn into the return line and created micro bubbles.