if the NEXT is louder, you may want to accelerate it slightly. there are several narrow speed ranges where the pump is more audible. Spinning up the pump can get you out of such ranges.
remember there's two speed pulses per rotation on water pumps. your other pump is effectively spinning at 400 RPM. there's not much flow there
As an example (it varies depending on the case, what mounting dampeners there are etc..) my D5 was loud at 50%, somewhere around 66% and at 73%.
just moving a few % off these values rendered the pump almost inaudible.