Tonight I got into my car to move it and decided to go for a little drive to warm her up. The whole time a friend and I are hearing this very strange high pitched whinning that matched with my rpms. I have no idea what it was, and it wasn't too quiet, as he noticed it as soon as I did. I almost wanted to say it was the synchros but he said it was coming from the back of the car. Any idea what this could be?

New car or used?

Bought it new, has just about 12,500 miles. I've changed the transmission fluid to redline and plan to change the rear differential sometime this weekend. Change the oil myself every 3k miles.

Nobody has any idea what this could be?

Was it the same in all gears and RPM ranges?

Over probably 5k rpms you couldn't hear it due to engine/exhaust noise. At 3k it seemed to be the loudest. It happened in gears 1-4, I haven't gone higher yet. It was especially noticeable when downshifting, as it would quickly change tone/pitch to match with the RPMs.