clipping sound
 

I've just finished installing my system last week, and the first thing i noticed is that every time i turn my car off with the radio on i get a clipping sound from my sub woofer. funny thing is that its only the sub thats doing this, the front component are ok. I'm running a 5 ch hifonincs amp with 2 guage power cables. Could this be a ground problem or an amp problem, any suggestions?