I'm having exactly the same sound and problem - it just cropped up this weekend on a long drive. Sounds like an owl is trapped behind the dash or in a wheel well, and I can feel the slight vibration in the accelerator pedal. It happens when I hit 45mph, and was intermittent at first and now happens every time I hit 45.

I have it at the shop down the street to see if they can diagnose it. My money is on a wheel bearing. It sounds like from this forum the same sound could happen with the speedometer cable, but my speedo isn't acting weird.

I'll post what they come back with.