Mine makes a different sound in cold weather, under light load it sounds like a wheel bearing gone bad or something. No vibration, no degredation in performance, just the sound. I realized it wasn't vehicle speed related, but rather engine speed related, when I downshifted (or upshifted) to an rpm that was above or below the 1800-2000 range it would quit, regardless how fast I was going.
I quit worrying about it as it went away when the weather warmed up. Came back with a short cold spell, then went away again when it warmed back up after that.
I was told it was normal by a few folks on here that had the same thing.
Hope this helps.
Duane