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Hello, I have a Martin Parlor in the shop for a repair. It is equipped with a Fishman Rare Earth soundhole pickup (NOT the blend version with the mic, just the simple magnetic pickup), that works well, but gives a sort of hissing trail on the lower notes. It's a bit like a hissing shimmer reverb. I got it out by simply taping an aluminum bar to the pickup, thus changing the magnetic field somewhat. But I would like to know where it comes from. Because what's really strange is that I also hear this scratching sound when I move a screwdriver in the magnetic field above the pickup. The screwdriver does not touch anything, it just hovers in the air between the strings, and when it moves the scratching appears. Any ideas on what is going on?