I have a 05/AT/Navi and when I drive with my radio on, there's a soft ringing/buzzing noise that lasts for a couple of seconds every few minutes. It's not very loud, but I do notice it. I thought it might be some noise based on the Navigation recalibrating itself? I dunno. Has anyone else experienced this problem before?

 

What kind of cell phone do you have? GPRS? Might be your cell phone sync'ing up with the network.