I have a 95 s10 blazer 4x4, 4.3 with a 4L60E automatic.

I got a noise coming from the transmission that sounds like 2 small gears running together. Doesn't mater if in park or in gear. Speeds up when rpm of motor is increased. Kind of sounds like gears that might be running without much lubrication, or maybe plastic gears.
I hear it the loudest with a stethascope against the bottom of the trans pan. But, I can hear it when I first get under the blazer without stethascope.
Trans seems to shift and everything fine. No clunks, slipping etc.
Could it be a speed sensor? If so what do I have to do to change it?
Pump?
What do you guys think? You know more about this than I do.
Hoping to fix it before something happens and I have to rebuild the tranny.

Thanks a lot for the help.
Much appreciated.