There could be difference in the PWM control of these pumps. If so, the is no easy way to lower the pump speed. In my case, D5 pump is nearly inaudible inside the chassis even at 3300 rpm. It is mounted on EK Revo mount.
My chinese D5 pump runs @ ~1850 rpm at 1% PWM duty cycle.
Sadly even at 1800rpm, it's still audible (slight motor whine) which is audible over my fans. At least at idle.