We're pulling an old Avion 30 footer with a not quite as old Suburban, not the 51. We have pulled it with the 51 but it's hard to carry on a conversation at hiway speed. We're home now and the 51 is waiting patiently for its next trip to Seattle. Its transmission was getting noisy so a couple of weeks ago i pulled it and replaced all the bearings and stuff with a kit from Novak in Logan Utah. it is a good kit. I got a whole set of better gears from a guy on Stove Bolt (thank you very much) and it's now much quieter.
I finally listened to your recording of the noise. It's not coming from a flathead, Stude-six piston; that I can say. But you've investigated all the things others have suggested that might make a noise like that so I'm at a loss too. Try pulling the front main bearing cap as Hot Rod suggested. It won't cost anything except maybe a skinned knuckle. They are tight. The condition of the bearing might reveal a lot. The upper shell of the no. 1 main will sometimes wear more than the bottom. It did on mine, I think because the 2 pulley shivs are driving a big fan, alternator, and A/C compressor. I think on the later engines the mains are not pinned to the block like earlier ones, which makes inspecting the upper half about four hours easier.
