It sounds like your sump does not have enough water in it. Try adding more water to your sump.
Also, since you only have one divider prior to your pump, if the water is moving too fast it will cause air bubbles in that drop to the pump chamber. The pump will suck those in & cause micro bubbles. If the water is too low, the pump will form a whirlpool & suck air in causing micro bubbles. Try adding some water so that when the pump is running full blast the water is at a higher level in the pump chamber.
If adding the water doesn't help, not sure what to tell you