A torque converter has alot of belts and small "clutches" per say. They rely on those belts moving to let fluid move in/ around the converter. At the stall rpm, ex a 2500 stall is when there is no slippage and the engine crank and trans input shaft is spinning at the same exact speed. So basically, every converter is a "lock up converter", but a true lock up converter locks up in overdrive at ANY rpm. Like Bill81 said, A slipping belt could cause this sound, but if it was real bad, which if you keep driving, and this is your proble, it will get louder. The only screen I can think of would be the trans fluid filter. And, if thats clogged, or your fluid is very old, the torque converter either isn't getting enough fluid, or the fluid is to thin to work properly. This will cause damage to both the converter, and the trans itself. Also, are you sure its coming from the trans, not the rear end? If this question is about your 94, everything I just said, for the most part should be thrown out the window because im talkin about a 350 or 700r4, not a 4L80E like your 94 has. So, what kind of trans is making the sound?
Best Wishes
zachh