My 2010 Ram 1500 RWD developed a new noise towards the end of a recent 900+ mile trip. There is now a prominent hum that changes pitch with the speed of the vehicle and is loudest from 25-50mph, quickly fading away above 50mph. I have checked for play in the wheel bearings (no play), there's no major wiggle in the u-joints, and rotating the tires this morning did not change the sound. The sound is present while accelerating, coasting, and decelerating, and does not change when coasting in neutral. The sound does not change when I turn the steering wheel or brake. I took it to a shop and they heard it but couldn't figure out where it was coming from. They did say there was some sort of a buildup on the brake rotor, but did not clarify what it was.
Here's a video of the hum in action: https://www.youtube.com/watch?v=5OUHrlI2uvQ
Could it be worn or misaligned differential gears, or bad carrier bearings, or something else entirely? The vehicle has 125k miles of which were about 30% towing, 60% highway, and 10% city miles.
Any help will be appreciated. Thank you.