Fully agreed that the sound is a result of my inexperience in piping design.
RD pump is a masterpiece of technology- Low heat, low power consumption, small, light and negligible vibration. Particularly all are good points.
As this is my first marine tank, I've zero experience and thought that 4 x25mm return pipes are sufficient for RD. But the sound is still too loud. If only my return are larger, I believed that the sound will be less audible...
I'm contributing in this thread is just informing the next reefer who intend to get RD to fix as large pipes as possible to minimize the possibility of this sound issue.