It might be a combination of palm mute between the pickups and the feedback created from the distortion pedals he's using, probably a combination of the Dunlop hendrix octave fuzz, ProCo turbo rat, and Sovtek Bigmuff... but sometimes he'll actually push the strings in toward the body at that position such that the strings touch the poles on the pickup this causes a ping like sound and with distortion effects it is amplified.
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