1998 Ford Explorer Rattling sound coming from bell housing

I've got a 1998 Eddie Bauer with about 180,000 miles (4.0L SOHC V6). The other day I started hearing a rattling sound coming from underneath the engine. The best way I can describe it is it sounds like loose bolt rattling around inside the engine. I checked the tensioners on the front of the engine and it doesn't seem to be coming from there. I put on a stethoscope to try to locate the sound and it sounds to me like it's coming from the bell housing. I pulled off the starter and inspected the flexplate gears, which appeared normal. I hooked it up to a scanner and no codes were thrown. The engine runs perfect and it shifts fine other than going into reverse kind of hard, which it has done the last 50,000 miles. I was sure it was the flexplate, so I pulled the tranny the other day and the flexplate was fine. Next, I sent the torque converter out to be tested and that was fine as well. I don't think it's the timing chain since I'm not getting any codes, the engine runs perfect, and the sound doesn't seem like it was coming from either the front or rear timing chain. I'm hoping it's not inside the tranny, but I'm beginning to think it is. Any thoughts?