Mine was more like a growling rattle that sounded like tranny in the gears or something, but when I replaced the clutch it went away. I think it was the spring dampeners in the clutch disc geting loose making the sounds from the tranny really come out. It would do the sound at low rpms when almost lugging a bit when taking off. At first I had tried the Synthetic mix in the trans but that didn't help. But after the clutch it was nice and quiet. The clutch wasn't slipping or anything, I just changed it because of that noise, and for preventive maintenance. I did it at around 130k I guess?
