I mentioned the SWCG above. Those are small, champagne size bubbles. Maybe a picture of what you are seeing may help.
Air can only get in the system with the pump running on the suction side. If the air is only when the pump starts, that can be from a leak, typically in your filter system (oring, air release, etc) that lets air in when the pump is off, but does not let water out when the pump is on.