Audi A2 slight thumping sound when pressing accelerator pedal

When I press the accelerator just slightly I can hear a kind of thud/thump sound coming from the front of the car. There doesn't appear to be anything wrong with the car - performance etc is fine - I'm just wondering what it is causing it. It happens every time at very low rows, pretty much as soon as you touch the pedal. Once the car is moving it doesn't happen at all, just seems to when pulling away.

I'm not overly concerned about it but just wondered if anyone could shed some light on what it might be.