Noisy a904

I hope some one can shed some light on this problem im having.when im driving around 30-35 mph i start getting a grinding noise comming from the tranny,once i go over to 40 plus it seems to go away.When i lift my foot off the throttle or put it in neutral no noise.It up shifts great, no slippage,no delay when putting it in reverse, oil is clean just a very slight leak from the rear seal.It dosnt do it in 1st or 2nd gear only in 3rd,I had the car fitted with a new exhaust system due to it was rubbing in some areas and was in need of a replacement but now those areas are fixed i notice the sound a lot louder.Can some one shed some light on this problem or have experience this before.The transmission is std with a stock converter running behind a 273.Thanks