My wife's system consists of an Alpine head unit with a 750 watt amp driving a 10" sub in the boot area. Once in a while after shutting off the headunit the sub starts making a horrible sound like a low rumble. It so loud it makes your ears hurt. Only way to stop it is to turn off ignition or open a window. Opening the window releases air pressure causing the sub to silence. Rolling the window back up causes it to start all over again. Suggestions?

 

Sounds like the amp is still pushing low voltage to the sub when the head unit is off.
Any idea where they tapped into the "amp on" line? (usually a low power source and a blue wire)