I have a 2008 WR450F I bought used a couple of years ago and it has always had a noise I can only describe as a howl, when in gear, with the clutch out. When in gear, clutch pulled, the noise completely goes away.
It came with an after market clutch cover and it almost sounds like it resonates. I'll get a link to a video file set up soon, but if anyone has experienced the same or knows what may be causing the noise, please chime in.
The bike always has, and still, runs very strong.