O. K. How often does it make this noise? Jack up one wheel of the tractor and have someone turn the wheel and see how often it makes this noise. 
Have you drained the oil out of the transmission and final drives and checked for broken gear teeth in the oil? does your tractor have the magnetic drain plugs? I know they did not when new, but some were added later. Speed increases the noise and of course the frequency of the noise but you may want to take the hydraulic unit (or what ever your tractor has for a transmission cover) and visually inspect the gears. If the noise is only in fifth and sixth check the gear on the pinon shaft. Also if you do have a hydraulic unit and you have never had it off you will need to remove the long PTO shaft first. Happy Thanksgiving! more questions? John