The grating comes from the transmission, it's most noticeable in 5th gear and when the weather and car are both cold.
sounds like your synchros. Get it serviced quick if it is still under warranty
Go get your tranny serviced asap. Your gears may be going bad and you don't want that to happen while you are trying to get somewhere! My brother in law lost his gears on his way to Arizona from California and got to spend the night in the cab of his S10. No fun!
If your car is still under warranty from manufacture or after market insurance just have it checked. manufacture warranty is free because it maybe that is was damaged during production or bad parts. Take it to the dealership you bought it from and tell them about your problem. Mustang transmissions are very pricey.
Not sure if this is the same, but I had a Nissan years back, with the same problem. The 5th gear sprocket was actually missing a few teeth. It eventually ate up the main drive gear, and then destroyed some of the other gears. Needed major work because I waited to long to have it looked at.
