That is the one of the most bizarre ones I have heard or seen, but reminds me of a humorous one I fixed. We had a customer who reported an annoying rattle when he went over bumps, but that it only happened in cooler weather. Well, we thought, maybe there's a very slight mis-adjustment that shows up only when metal contracts from the cold. So we did a full tune-up, which it needed anyway, with special attention to the headset, bars and front wheel, as he said the sound came from the front, just below him (downturn bars).

I test rode the bike over bumpy sections of road, sewer grates, etc. not a sound. The owner came in, tried the bike and said it seemed OK, but it was a warm day, so we said to come back if it reoccured. Sure enough one chilly morning he returned to report the problem was still there, even as he rode up to the shop. I hopped right on but the bike was still silent. At this point I was getting frustrated but I had an idea.

"Does it happen at slow speed?" The customer said it did. "And it occurred right here in the alley?" Yes again. I then asked him to get on the bike and slowly ride down the alley as I trotted alongside. Finally I too heard the sound, and fixed it as he rode, with no tools.
I just grabbed the zipper pull tab on his windbreaker and the noise stopped. That story was the source of many jokes around the shop for quite a while.