Thanks guys, appreciate the advice. The reason I suspect the Sherman is because it "sounds" like that's where the noise is coming from when putting my ear up to that area then the main transmission. Yes, the noise stops when the clutch is pressed, and speeds up when the Sherman is shifted into high. As you suggested, I will take the top off the main trans and see what I can see, and will check the spring under the cover (not sure what that is but I'll take a look at the manual). Never been in a trans before but there's gotta be a first time :) The engine is back together and running great after a major overhaul so I guess I did that right! I did not try turning off the hyd pump, will disengage the PTO and see what happens. I agree with you that knocking is a strange sound from a trans but that's what I'm hearing. Three knocks then a pause then three knocks and a pause, etc. I'm thinking it's a damaged gear as you said Dean. But why is a mystery. It was working fine and all I was doing was backing up a slight incline. I suppose it could have broken a tooth that was ready to go already and then the tooth got jammed and broke more teeth??? Who knows. Time to start looking inside.
One other thought. If the main trans is in neutral, isn't the only thing turning inside just the input shaft? And if so, isn't it more likely that the noise is in the Sherman since in either high or low range there are gears turning? Just thinking out loud.
Last question. Where can I get a Sherman manual? Don't believe I have ever seen one.