Ya, the volume required will have that unit cycling frequently. I went for a 75 gpd unit but "future business plans" will reqire higher volumes.
This unit came with a needle valve that can be fastened to an existing water line and pierces it. I like "make work" projects so I did some cutting and soldering...
Regardless of the valve controlling flow into the RO unit it shouldn't be howling I wouldn't think. My experience in Construction Equipment hydraulics leads me to think that the pressurized water is howling as it passes through an orifice. I dunno', perhaps the inlet valve is "starving" the unit of required water. That would be more like a "cavitation" scenario. I suspect the problem might be downstream...?
Any luck trying to determine the whereabouts of the noise ?