I had the volume at 0db, and I had already tried using only the front or rear outputs instead of all 4. When using all, more feedback is noticeable. When using only 2, there is less, but not much less.

Also, I believe this feedback is separate from the noise that I'm hearing when the engine is running. The hookups mentioned above were done with the engine off.

When the engine is on, there is also a noticeable whine that changes pitch with the engine's rev. But this is not noticeable until the cd player's volume is turned up to a certain level.