Id bring it right back to the dealer, it should be covered still. If I had to take a guess it could be something to do with a speaker ground. Ive seen that happen and it picks up in noise when the engine speeds up cause of a bad ground. You end up hearing the alternator frequency. Put it in park and rev the engine up and see if you hear it. That will tell you if its something thats driveline or engine dependent. If you only hear it when its moving then maybe driveline.