Annoying noise need help tracking it down

Hi

I'm getting a really annoying drone sound when on the motorway it sounds a bit like an airplane taking of and is more audible when I take the foot off the accelerator and let the car slow without braking. It's been at my local dealers twice and they have not managed to find it. At first I thought it was an airbus vibration but the dealer has fixed this. Anyone have any ideas its defiantly a windy/induction type sound.