I have had two distinct noises from the rear seat of my '17 MDX. The first was a defect in the reclining mechanism that would clunk over bumps. The second was a creaking sound from the right rear seat when the body flexed or I went over bumps. That one was solved by loosening the bolts and re-torquing them. But the sound was different than the one in your video. Yours sounds a lot like the creaking/squeaking of the springs or metal supports within the seat itself, rather than a mounting point underneath. Hard to tell without being there in person, but that's my first impression. Does the sound change when the seat slides forward or when it's in the "get in the 3rd row" position? Does it change when the seatback is folded over? That will help to eliminate things like the seat rail/mount or the reclining mechanism....