Okay, so I'm now (including the one from stock) on my third rear differential @ 51k. This is an 08-SRT8 first year Challenger. The replacement is howling @ 40-45mph, above that, nothing, below that, nothing, the older diff did this too, which they replaced under warranty maybe 1000 miles ago. What else could be causing this? I generally drive with the windows down and the music loud, but with windows up and music down I can clearly hear a howl from the rear, I'm just NEVER taking it back to the dealership that replaced it, they have screwed up just about every repair in one way or another. I was just trying to figure out what else COULD cause a howl from the rear between 40-45 mph.