Its an electronic buzz, have looked at similar topics on here and it may be a grounding problem but not sure how to proceed.
probably there is some dirt under the piezo saddles which prevents a good electric ground contact..
the piezo saddles are simply fitted into their bridge slots and are connected with small wires to the circuit under the bridge..
the length of the wires is not much, but it allows to separate the piezo saddles from the bridge slots if the latter have been unscrewed from the bridge..
what you can do is to clean well all the piezo saddles, especially in the lower part, which is the main point of ground contact, and of course you should thoroughly clean also the bridge slots.. and since you're at it, clean well all over the bridge, including the screws..
also, when re-mount the saddles turn them 180 degrees so the point of contact with the strings will be almost like new..
obviously this whole operation requires extreme care and attention, especially being careful not to break the piezo wires..