pictures of your set up would help, and a description of how your return and return lines are set up are sorta needed here.
Is your return pump in an area where there are bubbles? Is it close to your skimmer? Pictures...
I will try & upload some pics. My return lines run from a back room, not where the tank is. Yes the return pump is near the skimmer, but shut it off this morning for 2 hrs & still getting them.
There are some pictures or I should say, I hope they work for ya & want to say, Thank You for the reply!!