CR, this is an effective product. You can paint it on or use a spray can they sell. http://www.quietsolution.com/html/quietcoat.html
The rattles are vibration tuned to the frequency of the noise. Adding mass changes the frequency that the surface vibrates at. Some of the noise energy gets converted to heat as it's absorbed, but you wouldn't notice the temperature change.
The other solution is just to add some foam between parts that vibrate to dampen the rattle.