Then here's what I think is happening:

The RF field emanating from the FastPass/RapidPass detector is creating a current on the tape adapter wire as your car moves through the field.

If it bugs you enough to want to fix it, you can get a couple small ferrite chokes from Radio Shack and attach them to both ends of the wire to filter out the interference. OR, you can try re-positioning the wire in your car so it is less susceptible to the field. I'm not sure whether parallel to the direction of travel or perpendicular to the direction of travel (or somewhere in between!) is best. (Sorry, it's been a long time since I had to do any EM Physics. )