Help Identify a Sound System Problem Options

[Chovies]

So i will start by saying i have no idea what im talking about so what i describe now is about the extent of my knowledge

During the last month while playing music i would get a really loud pop/bang which sounded like it was coming from the left side of the car, slowly i had to turn the music down to avoid the popping and now im sitting on 8 when my normal volume is 20 and i still get this popping sound, its always random and is almost always when im driving/ cornering/ bumps in the road. I just had my car parked in the driveway pumping out 20+ and i couldn't get one pop.

The guy who owned it before me was really dodgy in his work and Ive noticed that the wiring is wrong in the amp ( front in (L) seems to control the sub while both the rear in inputs do nothing ) so im not sure if its dodgy wiring or my sub has blown or a bad earth or....anything...

I don't really want to spend my money on buying both a new amp and sub so if its something simple i can at least try then i may be able to buy one or the other.

Anthony