I wondered about the same thing and did a search for transmission noise. I learned that some noise is normal with age and the first thing to do is do an inspection and check for anything obvious like bearings falling apart and bad
CV joints and boots. Do a test drive and jack the car and feel for hot bearings.
If it appears to be wear related noise the consensus seems to be empting the differential, checking the drain plug magnet for unusual metal bits, and refill with redline 85W140 synthetic gear lube.
It works because its much more viscous then standard 75W90 gear lube when running. But because its synthetic it doesn't turn to pudding when it gets cold.
Redline oil is a speed shop thing, I couldn't find it locally and ordered a brand called Royal Purple. If you try this just don't try it with petroleum oil only synthetic.