First thing. Your transmission, are you awear that in neutral all rotating parts (gears, imput shaft and counter shaft are rotating. The only thing that is not turning is the pinon shaft shaft. Also check to see if the over-under unit sprocket is in alignment with the transmission  sprocket. Also check to see if the imput shaft is loose in the bearings. On the reply that mentioned transmission oil comming out of the bell housing. They made a change by putting a seal at the back of the imput shaft so that may not at all be a factor depending on serial number.  Have you checked transmission lube pressure? Also we had some that were noisy in overdrive at lower engine speeds and the problem was the PTO shaft was rubbing. do you have an Oliver shop manual? Questions?