If it help and you have a stethoscope around put it in different places and try to locate the noise. I did it once and I could hear the mechanical gears running but it did not help me fix itI am sorry you are going through this.
I agree with woodchucker. Drain the oil and run it to verify where the noise if coming from. It may very well be correct that it is the large gear but I would hate to go through the repair and not fix the issue.