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Dual sump fuel tanks have to have a non-motorized pump system for one side. There is fuel pumped over the to the side w/o electric fuel pump that goes through an orifice in a venturi (usually called an ejector) so that it picks up the fuel and that is delivered to the primary side with the electric pump. If there is no fuel then none is picked up. This happens the whole time the engine runs, it is not fuel level dependent. An electric pump can't be run dry so this ejector pump system is the solution. It also serves to mke a fuel reservoir for hard cornering as the primary pump is in a cup. The off side fuel(if any) is dumped into the cup. There is an ejector pump gizmo on the bottom of the cup also as keeping the cup filled until it runs over keeps the primary pump cooled and it is quieted a little bit. The fuel being sprayed through orifices in the ejectors hisses and can be heard if you're listening outside the vehicle. Noise level varies with fuel level and is usually loudest at below 1/3 gauge level as the off side is dry at that point and the off side ejector can be a bit louder then. Some versions use two electric pumps, one for delivery to engine DI pump and one only for fuel flow to ejectors. The other version has a diverted flow of fuel from main pump discharge that is the flow for the ejectors. The ones with two pumps can have a hum/whine that is barely audible since two pumps running is louder.
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