I have a cassette-only JBL system. Whether using cassettes (not an adapter) or using the radio, there is a noticeably high noise floor. In between songs in any moment of silence, it sounds like a little white noise. Think those crappy boom-boxes from the 80's-90's, you know it's on even when it shouldn't be producing any sound. This is with the car on or off, doors closed and lights off. I've used the same cassette in my Mercury and there is almost no noise from the cassette, both using Dolby reduction. Now would be the best time to troubleshoot, as I've stripped the interior save for the dash.