I discovered this thread after doing a search for information on T56 noises. Mine is a magnum that had a 4th gear whine from the day I installed it (new). I recently sent 1-4 gears to Liberty's to be faceplated. When I got the gears back I put everything together and now have about 500 miles and a few 1/4 mile passes on it. Right after I got it back in I noticed a slight vibration noise at idle that got more noticeable as it warmed up. At first I thought it was header tubes vibrating or something lose. Now that noise has become a lot louder and today it actually concerned me.
Another thing is, even though it is faceplated, 4th gear grinds and is not easy to engage. I plan to call Brad at Liberty's tomorrow but thought I'd see if anybody here has any ideas. I'll probably pull it apart next week and check it out.