Knocking and vibrating above 50mph

Hi guys, My Audi TT mk2 2007 (auto) has recently started acting up on the motorway, with a strange quick thudding sound with vibrations that shake the car. If I ease off the accelerator then the noise ceases, however, at 50mph or above the noise eventually stops after a while but always returns later on. I really hope it's not a rod knock or something serious. Has anybody got any idea what it could be?