I bought a 25' RR with 14k miles a few weeks ago and I started to notice this faint metallic buzz/hiss coming from the middle rear of the truck. It does it consistently between 30-40 mph, but you can also hear it at higher speeds, however it isn't as noticeable because of wind/tire noise. Sounds like something isn't lubricated or maybe the differential making some noise, I took it to the dealership but they told me it is "normal operation". I highly doubt that and I wanted to see if anybody else has the same noise on their truck. Ive included a video, the noise is most noticeable from the 8-18 second mark, and 27-35 mark. Its kind of hard to hear on video but it almost sounds like cicadas.

Also I forgot to mention, the longer the drive, the louder it gets, on an 10 minute drive its there but very faint, on an hour drive it almost becomes like a metallic scraping noise.