Motivation

So when I unlock my clk 430 the engine compartment starts to beep at 2-3 second intervals. However when i start the car it continues for a few minutes ( while driving) then ceases.What would cause this and why? It sounds like it is coming from the alarm siren but is not as loud and is a different speed and sound then the alarm. The alarm is not going off but I don't know what is happening. Someone please help me this is very embarassing and quite irritating to boot.

Please help me guys !!!