This noise just started on my 2008 AWD Limited. It is a higher frequency whistle/hum and it always happens around 40-45 mph. It doesn't get louder though, stays pretty constant. At about 50mph it disappears or blends in with road noise. Not sure if this is a fuel pump thing, was also thinking rear axle or bearing noise? This thread is pretty old and I'm curious if you've resolved this problem yet?