sometimes when i turn on my stereo or turning off the car.. i hear a sound coming from the subs i think. It kinda sounds like a light "boom boom". Does this mean there is some kind of distortion or interferance?? Also i've had my friend turn on/off for the sterro for me a few times and i've notice the sub moving very very slightly when he turns it off and on but no sound.. is this normal?? please help me out! I am running stock deck with a line convertor for rca wires to amp.