Subs pop when volume set to 0
 

My subs pop when I turn my volume down to 0 (also when car turns off). I can switch the volume from 1 to 0 and I hear a pop. I have a pioneer head unit , fx700bt. The deck sometimes loses all power and turns off, randomly.. I don't know why, sometimes it's when the music is Loud, sometimes it's when I just push the back button. Is this a grounding issue? With the car turned off I went around and checked all connections with a multimeter to see if I'm getting 12v to all my amps and the hu. It was fine.. How else would I check for grounding problem?