Sorry, didnt mean to freak you out or anything. Its hard to diagnose a sound by description without hearing it. The 4.6 is a noisy engine though, even stock.
You should hear one when you crank one up for the fist time...My god, you want to run and duck. They make some hellacouis noise for the first few seconds. Takes some getting use to....and I havent heard enough yet to get use to that.
Anyway, best bet if you dont have alot of automotive back ground yet, take it to the dealership when it doubt...esp since its still underwarranty. AND Especially since the 3 year thing is coming up here shortly for you.
Sounds like a nice car you got your self there. Get ya some gears, an H-pipe, a pair of aftermarket Mufflers (I'm partial to FlowMaster) and some good tires on all fours, and you'll think your driving a totally diffrent car.
One thing you gotta love about them Bullitts....The seats...I love them. The look, and the feel....sweet.