Many Challenger owners have complained about rear differential whine. Some have changed the gear fluid to Royal Purple and others have added thicker oil. Some have even had the differential replaced under a recall. Unfortunately, all of these seem to be temporary fixes and the whine eventually returns. I have found a very plausible explanation for this long standing problem that goes back to the classic Mopars. Our cars have an independent rear suspension and the differential is mounted to the chassis through rubber isolaters instead of being suspended by springs on the ends, like cars with live rear axles. As a result, any noise is more likely to be transmitted to the driver's compartment. Unless there is metal in the differential fluid, and nothing is clunking or broken, it's just an annoyance. (Note- If you install a differential brace, you will notice it more. This makes sense since it is another connection point for transmission of sound vibrations).