The Belkin TuneCast 3 FM transmitter's car power adapter has an unusual thickening at the plug which plugs into the transmitter. I suspect that is a filter which prevents the transmitter's output signal from entering the power cable and I theorize that if the filter wasn't there some signal would radiate from the cable resulting in a stronger output. Sure enough when I used a 5V adapter I have instead of the transmitter's adapter (75mA at 5.5V, centre positive) I got a noticably less noisy signal. It might actually make the difference between the FM transmitter being too weak to use and it being usable.
Note: I'm not recommending this FM transmitter. I am just using it because I was able to buy it at a very low price and I only need it very occasionally. Yes, it's probably the best looking FM transmitter and the multi-colour OLED display is impressive. However, the most important thing is being able to hear your music without noise, and it is not very good at that. Some people also have problems with it shutting off automatically. I didn't have that problem.