I give up.
Have a dual piezo based three string with bone nut and bridge. Has a hum when plugged in, strongest when I touch input, so I replaced potentiometer, input jack, grounded pot to itself and to jack and still has slight hum. Compared it to several others I have and they are very quiet. Any ideas appreciated. Have not grounded the strings but I was under the impression piezo setups do not need strings grounded, plus not sure if it matters.
My experience is that Piezos are very inconsistent in the noise department. I've used some that are very quiet and others (in the same type circuit) be very buzzy. Can't say I know why.
One thing I did notice is that a Piezo will be noisy with a low quality cord. A decent triple insulated cord seems to work best and not cost an arm and a leg.
The other thing is the amp. Some amps are noisy with a piezo but quiet with a humbucker.
Odd as it sounds, try getting the hum going and reach down and touch the metal of the output jack. It may immediately get quiet. If so, then ground the strings might help since your fingers will be in contact with the strings most of the time.
It really comes down to methodologically determining what make the hum worse or better, then using that info to improve the guitar.
Try some of the above and see what you can learn.
I have two piezos wired, is it possible one could cause this?
Try a temporary placement of a.001uf cap across the jack or pot and see if that helps, that is if grounding the strings doesn't work. You may lose a little of the hi's, but you will eliminate a lot of trash noise.
Check all the solder joints for good solder flow, if they have a gobbed appearance they aren't good. Could also be your cord.