I get a good feeling that is what is going on with your vehicle & sound system. You described your problem pretty well. Just make sure all of the wires are swapped out. That includes the wire to the distributor too. By what I meant by "insulated" (these wires are all insulated) is for radio. Almost standard these days, but you might want to clarify when buying.
I have had breaks in it, such as you described, in the that you could actually hear the buzz picked up from power lines when driving under them tuned in to AM radio. All of these power sources (engine spark system, power lines, etc) readily broadcast radio noise that your system can readily pick up & play back to you via radio.
For the 'what is worth' category since you have already done this, a poorly grounded sound system (either the unit itself or the speakers) will buzz regardless of whether you are playing radio or a CD. This is why you saying that it only happens when tuned into radio was very helpful towards giving some advice back. Also, non radio insulated speakers of any sort (could be your PC) will also intermittently report a "dot the dot, the dotta" sound back too from your cell nearby cell phone if using the frequency that ATT and other carriers that use (will not happen with Verizon and their brethren - they use a different frequency).
All of this is from the "have had to deal with this 'crap' department!"