When I bought my current '65 C20 in 2015, it made a noise that drove me nuts. I replaced this and that with parts I had on hand, and it still did it. I deduced it down to transmission bearings. More so, the front bearing. Rebuilt the transmission, the kits came with new bearings, and the noise was gone. The noise did go away when the clutch was depressed, but was there when driving, idling, coasting, as the input shaft kept turning. Depress the clutch pedal, the input shaft stopped turning and the noise went away. Granted this was in a Saginaw 3 speed, not an SM420 or anything heavy duty, but you should be able to get replacement bearings for yours.
