I was sitting on the deck watching the pump and such work (hey, we finally got it all working and no leaks, so I'm happy). So I was sitting there and I heard a burbling noise, I ran down to check on everything, pump ok, filter ok, pressure ok (16 psi), no leaks. But there was a string of bubbles that came out the discharge from where the pump returns the filtered water into the pool. Only happened once. Any ideas on the cause or is this no big deal?