So was it any easier to discern where the noise originates from without all the exhaust racket? If it is in the back there are not many options. Flywheel rubbing on something maybe. If it is inside the block I guess you could go for it and use the hope for the best method. Most any idea I could come up with now would border on speculation. Maybe the camshaft is working its way out the back of the block. Probably not if the distributor is ok. I'm mostly at a loss right now. Locate the noise and you will more than likely have it figured out.