So very stuck...

I thought the same thing with my 69 D200. It started rattling faintly, then under more acceleration it got louder and worse under full throttle. Found out one of the bolts that held the converter to the flex plate had come out completely and two others were on their way out and hitting the block! I didn't figure this out until I was pulling out the engine and trans because I thought it had spun a bearing, that is exactly what it sounded like! Do a quick check to be sure before wasting time like I did...