I would agree on checking the shock towers real good. Look at the tires from the front and note if they tip in at the top, if so the towers are starting to tip ( bad news). Take the car for a test drive and take it up to 55mph if posible then put the brakes on firm. If the car pulls hard to the left or right the towers maybe tipping. I have had may mustangs in my years and most have had this problem overtime. Take your time and look it over real good.