I went to the airport to tinker with Randy's wiring again.
There was a barbecue going on in Ken's hangar, and Mark (who I flew up to Auburn a while back) showed up in his mint MGB.
Mark took me for a little ride in the thing - it sounds great, especially as I'd driven there in the almost silent Bolt :o)
After lunch, we peered at Randy's wiring - we removed the inline filter we'd installed at the weekend, as it's not helping. Ken reckons it's the audio plug causing it, and suggested using a mono plug instead. Testing it shows this works, but I still don't get why it being powered up causes the noises. Whatever, Randy's ordered a new plug.
My DHL package arrived, and luckily Vic was there to receive it.