I had a subwoofer installed about 6 weeks ago and it sometimes makes noise even when the radio is off. It is a really loud deep sound that gradually gets louder. When it happens it is usually right when I turn the car on, because I shut the radio off before I shut the car off. This problem occured the day it was installed, then it was fixed up until now. Why would it all of the sudden start doing it again, and what causes this, and how do I fix it?