So what your saying is it only does it when your lugging it. 35-40 MPH you should be getting a littler higher up in the RPM range. Mine make a kind of "growling" noise if you will when I'm at low RPM's in 4th. Also I swapped in a fully splined main shaft in my trans and I had to use a gear puller to pull off the 3rd-4th synchro hub but when I went to put it on the new shaft it just slid right on. Didn't have to press it on so I'd say your fine there. Did you check your end play when you were finished??