To be honest I am not a John Deere man, but replied based on my experience with both open and closed hydraulic systems I have dealt with over the years.
A 4020 may have more than one pressure regulator valve governing seperate parts of the hydraulic system.
It sounds like when you aren't diverting part of the pumps output by using the hydraulics that is when you hear the noise.
It might also stop the noise if you were to connect a remote cylinder to one one set of your remote ports and then move the cylinder which would also divert some of the flow momentarily stopping the noise.
I assume the noise is worse at only certain RPMs and is most likely the worst at near idle ??
I would think that somewhere the spool, the spool bore or both, might be slightly worn.
Hydraulic fluid, like water, is not really very compressible and when the gears in a hydraulic gear pump are pushing against high pressure it creates tiny pulses that resonate through the hydraulic system.
Every hydraulic system has a certain resonant "sympathetic" frequency at which the pump pulsations will create noise and vibration much worse than at a RPM that is slightly higher or lower than the resonant frequency.
Every hydraulic system is different and some don't seem to be noisy at all and others sound like they are about to shake themselves apart.
When you divert a portion of the flow causing a momentary slight drop in pressure the pressure regulating valve stops having to regulate and the noise stops.
Where the valve is I don't know, but that is where I would start.
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