Sounds like an issue with your cassette deck. You should definitely be getting better audio quality with a cassette adapter, mine sounds better than a normal cassette tape. Every radio frequency transmitter I have ever tried has had severe artifacting on the high end (treble), especially prevalent on "S" sounds; they come out really static-y. At any rate, your better off doing literally anything else than repairing your cassette deck if it's cooked, but its worth a go to try a standard aux cord cassette since they're under $10.

I've actually opted to record my playlists onto cassette tapes, and I wound up using these more than the aux adapter.