1. Free possiblilies..check your connections, grounds and positives..May just be a loose or corroded connection.
2. Have someone listen to it with you..like Chad
..don't drive much w/o knowing what it is. I'm thinking either pinging (predetonation from bad knock sensor) or something you don't want to hear. Check your oil and coolant levels right away also.
3. If you recently purchased the tires or have free road hazard..have the balance checked. Usually alignment doesn't go out of whack without being whacked very hard.