Tap, tap, tap, knock, knock

[X5chemist]

Recently a tap started in the rear. You can hear it only at low speeds. The slightest movement will cause it. I drove the rear up on some 4X4 wood ramps. I used a big pry bar trying to find the tapping source. It's not a suspension part. Not the battery. I even took out the back seat. I found a quarter.

I checked play on the driveshafts. Nice and tight there. The tap seems to be above the differential. Behind the seat. Cargo forward area. It's like something is rolling around. Or loose. It should be obvious. I can't see it though. Next week, I'll remove the spare tire and associated tools. With everything out of the cargo area, I may install the hitch too.

Any ideas on what's causing the tapping noise?