John is right on the money. I had a similar situation last year. It drove me nuts till I discovered there was a small piece of debris that was stuck to the O-ring of the pump basket which allowed just enought air seepage to create the bubbles from the returns.
I see the bubbles primarily when they surface. I reduced my chlorine setting on the SWG from 40 to 30% today and checked on the bubbles off and on. Interestingly, sometimes they are there, and other times later they are not. Is there any way to know when the SWG is generating by looking at the aquapod display?