I've got an '01 XJ, AW4, 4.0L w/158k miles and I have a similar, but quieter, sound only at idle. At first, I thought it was rod knock, but I think I've traced it down to near the rear of the engine and maybe in the bellhousing. It's definitely not as pronounced as the noise in your videos, but it's somewhat similar.
Anyway, here's to hoping someone can figure it out...