Well... my buyer has sold it on already. Seems he doesn't hold onto gear very long. He did, however, assure me that he wouldn't have sold it on if it were still making the static noise. He looked around for a likely culprit in his house and apparently found an ethernet over AC adapter that he'd forgotten about still plugged in. Once that was gone, I think all was good. Thank you, everyone for all your help and advice.
