Radio noise or Feedback

I’ve had my 09 TCH for about two months and have picked up what I could only describe as a faint feedback or noise sort of interference that seems to be coming from the speakers. The radio doesn’t have to be on to notice it, in fact, it is easier to hear with the radio off. It doesn’t happen all the time but it does occur predictably when I drive through certain areas. Problem is, as luck would have it, that none of these areas are near the dealer so I am having a hard time with him hearing the noise. They have ran their diagnostics and tried to find the source of the problem with to no avail. My question is if anyone else has had this problem and fixed it, could you tell me what they found so maybe I could point my dealer in the right direction? Thank in advance, Rick