When you say "water comes out of the pump" do you mean water comes out of the waste port on the multiport valve?

Does your multiport have a recirculate setting? What happens with that.

Is air getting sucked in through your skimmer into the pump strainer pot (pump skimmer)?
Is air leaking into the pump strainer through the lid?
Is air leaking into the suction line at a fitting?

You can test for suction leaks by smearing some shaving cream onto the suspected joints and watching to see if it gets sucked in.