Mine does the same thing - 72 untouched as well. Not having much experience with working on transmissions, I took it to the shop. The mechanic took it for a 10-min spin with me and said that the sound is caused by the gears getting pitted with age.
He said it isn't really a mechanical problem to worry about, just annoying.
Now he could have been full of sh*& but you'd think he try to grab some money out of my wallet if he had the chance?