When I put the new Python exhaust into Blitz., a droning noise krept in that wasn't there with the old exhaust. After some mucking about I worked out the rear bumper rings like a bell when struck. The new exhaust extractors run parallel to the bumper so easy fix. I applied sound deadener to the inside of the bumper. Filled the tank went for a drive, problem solved. On a trip recently a small amount of the noise was still there. Because I was sensitive to the original droning it anoyed me, but Kym couldn't here it. And then the penny dropped. When I filled the tank the noise went away completely. At about half tank the noise came in arount 80kmh went out about 90kmh, louder as the tank got lower. The tank came out of Blitz so long ago in the restoration, I can't remember if there was any isolation involved. Rest asured there will be soon. I don't know if anyone has had the same sort of problem, specially if you've used sound deadening material to keep noise down. The sound the harmonic in the tank is like drone, drone, drone, drone at about 80kmh. almost like a trans noise but softer.