Hmmm, that's not what I expected you to say. 3rd and 4th gears are on different synchronizers, so I'd rule out a synchro needle bearing as the culprit. How about putting the car up on jack stands, running it in several gears, and listen at various spots with a stethescope? Try the output flanges on the trans, and also each rear wheel bearing. Maybe that will help narrow it down. Or what I usually tell my friends - bring it to the phone and I'll look at it
. -Steve