I had more of a 'whining' noise on my old '03 4200. It came in at 80mph. The garage decided to replace the gearbox under warranty which fixed it. Your noise could also be something to do with the induction system - maybe sucking air in through a place it shouldn't. See if you can get the noise when stationary with the bonnet open.
I had an intermittent whining, usually it would start when I was stopped and it would continue throught the first 50 feet or so of acceleration and then stop. Dealer determined it was the gearbox which was replaced under warranty.