WTF is this sound, pinging?!

The first thing I would do is try a tank of premium gas and see if it goes away (I'm not saying this is the solution if it does go away, it will just narrow things down). If it does it is probably detonation (aka pinging). No matter what take your car back to the dealship to see what's up. If it is just detonation the car just needs to be retuned (hopefully). I've had two mustangs (5.0 and 4.6) and both did not like 87 octane, especially the 5.0. I would run high quality midgrade to save money over premium gas and found out that my cars ran fine on Texaco midgrade (Texaco was the best, both cars never pinged on Texaco midgrade, Amoco the worst, always pinged on their midgrade) and the pinging would go away. I'm a bit concerned that you hear this all the time and not just under hard accelleration. Good luck and let us know what happens.