Roms,

You didn't say whether your music system is on or off when you turn the car, or does it pop in either setting?

My cinema surround speakers used to pop when I turned on the juice, but a Belkin surge protected lead sorted it out. If you don't have anything self-installed which is electromagnetic near the speaker, then you are probably sending an electric current to the speaker magnet, which causes the pop, when you start the car. Since it only does it once, the car is not sending the current repeatedly. Dealer should be able detect what is sending the current and ground it out for you.