In reply to Philip:
Sounds to me like the hum could be the pressure reduction valve not working well under light flow conditions causing the pressure & flow to fluctuate rapidly, chances are once your tap is wide open the regulator is seeing <3bar at the outlet and is wide open and not therefore resonating.
3bar>
It's also possible the valve performance is normal but somewhere in your system there's a pipe-run or bend that vibrates noisily under fluctuating flow conditions. Fillers on cisterns are the usual culprit for driving this type of banging/humming noise as the flow varies with the ripples on the cistern surface before closing off completely.
There are plenty of ugly joints in the average plumbing system, I'd be surprised if the step from 22-15 or back is driving your resonance.
Surely whoever installed it should be willing and able put it right, it must be a common enough problem and I presume you weren't quoted for musical pipes!
jk
Post edited at 16:20