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).
Doubtful that the IRS and rear cradle are at fault. I've had many vehicles with that same setup and they didn't have any noises from the differential. Differential whine such as we experience is generally caused by improper setup of the gear sets and/or improper lubrication. It's also quite possible that the actual gear sets chosen for our cars is the culprit. Different designs (mainly gear cuts) can offer varying levels of performance and durability but can also have varying degrees of "normal" noise levels. Occasionally, components chosen for certain heavy-duty applications bring with them compromises in NVH (Noise, Vibration and Harshness). We had such an issue many years ago in the limousine industry when Cadillac spec'd the heavy-duty truck rear for a commercial chassis used for making stretch limos. It was more than up to the task of carrying the loads but since it was designed for a truck, it tended to be kind of noisy for a luxury application like a limousine. Bottom line, unless we can get direct communication with Dodge engineering, we'll probably never know why our cars occasionally whine but since I haven't heard of any widespread failures I'll assume that the noise isn't anything to be concerned about.
Maybe YOU don't but I've fixed plenty over the years. Again, my guess is that the whining occasionally heard in Challengers is possibly due to the way the gear set was manufactured and therefore could be considered "normal" for the car. Chances are good if you could find someone well versed in these particular differentials, you could find your answers.
If it is the gear nest assembly, would they not eventually quiet-down as the gears begin to wear into one another? Back in the day, we use to run non-synthetic oil during break-in on our HD big-twins, then switch to synthetic once break-in was complete. The idea was that synthetic oil would prevent proper break-in. I'm not sure if this was any help, but we never had any issues with this process.
Many applications stipulate NOT to use full synthetic oil during break-in. At least there used to be. Is it "possible" that the Challenger utilizes this strategy? I suppose it's possible. Could it also be that the gear sets making noise simply haven’t broken in yet? Again, I suppose it's possible. I know mine made a decent amount of whining for the first 1000 miles or so but has been quiet since. Or maybe my louder exhaust just covers it up?Regarding the actual gear set, it's quite possible that Dodge engineers opted for a gear set that provided enhanced durability but required a compromise in noise. Or maybe some sets weren't properly setup at manufacture. Or maybe the lubricant used isn't sufficient. Or maybe the chassis/drivetrain design isn't optimal for dampening drivetrain noise/vibration. Could be a combination of any of these issues as well. Without inside knowledge from Dodge, you'll probably never know the answer. The one thing we DO know is that the noise is fairly common in these cars and best of my knowledge, nobody seems to have come up with 100% effective solution.
