For the last two days I have noticed a noise from my '51 8N. Started out as a high click (snappy - like an electric spark jumping a gap) Seems external to the engine and on the right side. Today it got progressively louder. Tractor runs good. Noise seems to be coming from the rear of the manifold or the manifold muffler junction. I cannot detect any exhaust pressure at that point. I am considering removing the manifold for a look see. Any thoughts??