Same thing can happen with XM satellite radios as well. I used to use the built-in FM modulator in my vehicle, but have since switched to a cassette adaptor.
I use Sirius satellite, i love the built in "transmitter" which i use all the time. I'm actually considering switching to " hardwire" to get a cleaner sound, i get "outside interference" from the so called "public " radio.
BTW I don't miss am/fm radio and their commercials and lame playlists. Satellite is the ticket with so many stations to listen from :)