I get the same with mine. It used to do it exclusively at 80 MPH and above; now, after I swapped out my exhaust, it's shifted down to 75 MPH and above. Probably just the exhaust - you likely don't notice it in other gears because you don't hold it at that RPM very steadily or for very long like you do at cruise in 5th gear - you pass over that drone-ish exhaust note tone.
If you increase your speed and/or RPM's, does it go away? If so, there ya' go; if not, then it could be a lot of other things. Rear carrier bearing of the tranny could be worn, U-joints could be getting sloppy, pinion bearing might be groaning a little ... and so on and so on.
Mine sort of has this really low subwoofer bass-like vrrrrroowwwwwrrrrr ... vrrrrrrowwwwwwrrrr thing going on over and over - more pronounced but more tolerable since the exhaust change, since it's now lower-pitched, and also leads me to believe it's exhaust-related. There's one "throb" of sound per second at 80 MPH, which is right about at 2400 RPM for me in 5th gear. It's done it from the first day I owned the car, it's never grown worse, and there's no metal shavings or burnt fluid or anything in the tranny or rear end, so I'm assuming it's relatively harmless after having had it for the past 25k miles or more.
