I played the o.p. video for my wife (blind) to see what she might think. Her ears are amazing and have certainly done their fair share of tracking down weird noises from tandems over more than 12 years. She says drivetrain, maybe brakes. The four mile delay before the sound starts really points to brakes and this could be tested by doing an insane amount of braking in a shorter distance, say 1 mile. The LBS might not have the patience (even at shop rates) for all that lead time so maybe the bike could be brought to them already warmed up, as it were? I don't know, seems to me swapping in a wheel from the shop could have quickly eliminated (or implicated) the rear wheel without the time (and $$) of a hub rebuild. Right at the end of the first video my wife and I both hear a gear shift take place and an immediate diminishment of the sound. Dumb question: what gear combinations are favorites? Are any of them cross chains? Our latest tandem seems to have no tolerance for much more than three rear shifts and there is no trimming possible with the indexed front derailleur. Almost the only thing left is the frame. Are there any plans to test the welds for soundness? I didn't catch the brand of the bicycle on the boom tube but ongoing costs of investigation IMO should be weighed against what a used model of the same type goes for presently on Craigslist. FWIW.