It sounds as though something is loose in the transmission and is vibrating (the noise you hear) when the brakes are applied and the load is taken off the transmission. Then, when the brakes are released, the load reapplies to the transmission (as the car begins to creep forward) and whatever is rattling is suddenly under load again and stops making the noise. This could be down to something in the transmission itself, and if that’s the case, could be a worn torque converter. This is the component that actually turns the engine’s power into a force that drives the transmission and, eventually, the wheels. Inside the torque converter is a series of vanes. If one of these is loose or damaged, a rattle can be the result. However, before you rush to that conclusion, have a good look under the car. There’s every chance the noise is a simple case of a heat shield, bash-plate or even part of the exhaust system rattling at a particular engine frequency. When you take your foot off the brake and the car starts to move, the engine revs change, the vibration frequency changes at the same time, and whatever it is stops rattling. An internally broken catalytic converter is also a prime suspect for producing a rattle at certain vibration frequencies.