2006 Volkwagon Jetta 2.5I strange tick sound

I have a 2006 Volkwagon Jetta 2.5I hear a strange "tick-tick--tickkktick---tick" sound coming from the right hand back side of my car. As if someone is knocking my back door with an inconsistent frequency. It usually starts during the "first acceleration cycle - after ignition". And goes away after 2-3 minutes. The sound is audible both from within and outside the cabin. And it doesn't matter whether the car is moving or staionary. It will stay for sometime and will be gone after I drive 100 - 200 meters.I'm wondering what this sound is all about.