Originally Posted by Rob J

My 2005 FX4 SCrew has slowly developed this similar rear end noise as well. I'm now at 91k miles, and it seems to be getting louder. Only thing is, it's more of a higher pitched "AHHHHHH' sort of sound. You begin to hear it at around 25 mph, and it gets increasing louder as speeds build. At highway speeds, it has now made what used to be a pleasantly quiet truck to now a rather annoyingly loud truck. (Have had the truck since new) You only hear the noise upon acceleration. Letting off the accelerator, or shifting into neutral, results in the sound ceasing until you get back on the gas. Other than this issue, this has been a remarkably great truck.