Sounds to me like your stock may be touching when you are on the gun and putting a little pressure on it. Those cheap plastic stocks don't take much to make flex and contact the barrel. By sliding the bag closer to the center of the rifle you are reducing the amount of stress on the forend so that it no longer touches. Thats my guess. I would inlet it until you can at least slide a business card down the barrel without touching. The better fix would be to upgrade the stock completely.