"Try to put an extra shield around the piezo section of your pickup if that's possible, using conducting tape.
Connect the extra shield to the shield of your wiring as close to the piezo as possible.
This could solve your problem.
"First you have to rule out bad connections and wiring problems.
If they hum more when you get close to them with your fingers you could shield the pickup with aluminium or copper foil and connect to ground. Piezo's are very sensitive for low…"
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