How can I inspect 6" horizontal ducts to locate a rattle?
I can't imagine needing anything more than a cheap borescope and a fish tape. The trick would be attaching the borescope in such a way that it doesn't collect gobs of lint as you work it down the duct.
I'd actually forego the camera to start and just knock a fish tape or light plumbing auger around a bit. Maybe you'll knock the thing into a more favorable position.
You could also work a powerful vacuum down there with hose extensions. Be sure to tape them together well to avoid a worse situation.