This is a long shot, but if you have a Nextel phone, there is a 98% guarantee that it's your culprit.
Try it out. Have someone call your phone (assuming it's Nextel) while you're in the car with the radio on. If you get a ch-ch-ch-ch in the speakers, you'll be able to stop going crazy.
The only other logical explanation I have, (again a long shot) is if you have the stereo on AM and your turn signal is on at the same time--you'll get a very similar speaker noise then.
hth
