Hey Paul,
I might be slow on things - If they repaired yours under the warranty, while they didn´t find the issue, what was the solution then? (recone, replacement of drivers), and do you have your repaired driver back in fully working order without those issues?
Was the original issue repeatable with free air driver, so B&C got better chance of finding it?
I had small ticking noise with 18SW115 due to terminal leads, and maybe something with 21DS115, but only under very extreme conditions, while it solved itself after burn-in or with slight bending of those leads. Nothing like "diesel engine" as you guys describe it. I´m so used to B&C quality that I might feel childish butt-hurt at first, when I´d found less than flawless product from these...
Anyway, I would blame the enclosure on first sight, but again, it´s hard to tell. You might need to do more detailed break-in procedures from now on, and break down all your steps between getting those drivers shipped and getting to the failure, to get to the issue.