Vauxhall Astra loud prang when accelerating

when slowly accelerating, to get out of drive etc, from still, in first gear theres a really loud pranging noise thay comes from underneath the car at the front drivers side. sounds like somebody has pulled a piece of metal back and let it go and its pranged back causing a loud twanging sound (if you can imagine it) can feel a slight vibration my foot when its on the throttle but nothing major, got the car under warranty for two months and the garage are insistent there's nothing wrong but i think they're lying as it could be major, any ideas please??