citroen c3 beep when I start the engine?

I have car citroen c3 1.4 HDI. 2008. when I start the engine after 1-2sec. it beep only once. Everything else looks normal, all indicating lights turn on and than off, start the engine is with out problem and work properly. The same thing beep is happening every time when I start the engine. I check all lights and there is no problem. Can someone tell me what is the problem? Thanks.