I wouldn't consider Scottsdale to be a southern town
Anyhow, I think I know what you are talking about, sometimes my car makes a sound like you describe, I used to think it was from the speakers, it's like a MMMMMM hum (but it has characteristics of being a vibration, reminds me of going over certain types of roads) at low speeds (usually under 20mph). It has periodically done this for 3yrs, and nothing is wrong, I'm at 42k miles. You can always bring it in and ask, it never hurts to take a ride with the technician.
Enjoy your BMW, the 07 328i replaced the 325i and I hear it has
almost the same performance as the previous generation 330i.