2004 Ford Mustang Shifter

I recently added a short throw shifter (MGW). When I was taking out the actual base of the shifter, I needed to wiggle it out side to side because of the RTV silicone. Well, when I was wiggling it out side to side, it somehow slipped into fourth gear without me noticing it, so at some point in time I was wiggling it in fourth. After I installed it, and went to go test it out on the road, I noticed that the shifter makes a grinding noise in fourth gear. The grinding noise isnt consistant, it only makes the noise in little spirts and usually makes the noise while im de-accelerating not touching the brake. It makes this noise only in fourth and starts making the noise when the car warms up. I also wanted to know approximatly how much it would cost to fix this problem if I sent it in to a mechanic.