Rattling noise

I have a 1984 pickup 350. I have a rattle coming from where my flex plate/ring gear assembly is. Almost sounds like a ring gear rubbing against my inspection cover. I took my Inspection cover off, that wasn't it, but I could get my ear a little closer to where the rattle is. I'm hoping it's not the rear crankshaft bearing. What kind of sound would that make, and how far is my crankshaft bearing from my ring gear?.....thanx