To be a little more serious than my last contribution (for which, humble apologies), I have experience this phenomenom a couple of times. One was something vibrating in the ventilation system, so try closing all the vents, the other was a sticker that the channel ferry company had stuck on the windscreen that only vibrated when the car had the right speed through the air. As this latter one manifested itself when I was driving down through France I was tending towards mild paranoia, what a relief when I found the culprit.