Eddie Hotrod

When I bought my '85 3.2, the gearbox was lovely and quiet, but recently I decided to change the lube and put in the famous Swepco 201. Now a get a sort of friction noise at low revs under load. I filled the box until it started to drip out, then let it stand a while (level) until the dripping stopped so I think the level is OK? Could the previous owner have put in an additive to stop noise to sell it, or have I just been unlucky? Is there something I can put in to stop the noise just until the winter? The noise doesn't really stop my enjoyment of the car, but it's bugging me that it wasn't there before I changed the oil.
Any help appreciated.