how low is the waterlevel, to the input on the pump? if the waterlevel gets too low, the pump can pull water from the surface, causing bubbles. also find sometimes ball valves can cause them, i have seen them on my smaller pump from the ball valve.
but i know if the level in my sump lowers too much, even though the pump still under water, it pulls water down from the surface, and blows microbubbles out into the tank.
i also had some issue with them from the gravity side into the sump, baffles fixed it, but i think the mechanical filter media was causing them there..
just something to look at...