Audio system noise at 30 MPH
 

My 2011 SRX4 just began a high pitch constant "whistle" noise when the vehicle travels right around 30MPH. It's all stations, including Satellite and CD player.
What could be the problem?

Any repairs prior? Does noise change with speed? If you use fader and balance can you isolate speaker? Alternator is a possibility, but I would have it looked at vs guessing