What causes a rattle in front drivers tire ?

Sounds more like a suspension problem. Take the wheel off and search for google image what they should look like for your car(though most cars suspension are very similar). And see if you can find something visually wrong.. if it's a major rattle you might find something odd immediately. As far as I know warped rotorts cause squeeking (pads coming into contact even when you're feets off the pedals).

that mechanic was right about a warped rotor. You really should changed your brakes along with your rotor. When you hit your brakes on a warped rotor, yes it will vibrate because either the rotor is old or your braking too hard ( stop sooner). Sadly, too many makes simply don't make rotors the way they used to because it's just another way to get us back in the shop and more money out of pocket. HOWEVER, a tired that isn't weighted / balanced right could also give you issues. A dodge charger is a high performance car, however, but they still cannot stop on a dime. good luck