Audio Systems: fli Active 12 amp not working, tight metal, bulit

Question
i have a fli Active 12 sub and bulit in amp . i have fitted it and it worked fine then i took it out to put something in the boot when i refitted it i can only turn my gain up 1/4 of the way if i turn it up any more then that it starts pounding on its own even with the music turned down , any 1 no how i can fix this

Answer
I think you have what is called "motorboating" - named after what it sounds like.  Is it a low frequency repetition of a blurping or barfing sound?

If so, it is almost always, say 90% of the time, a grounding problem. I suspect when you went into the boot you lost a good solid grounding connection from wiggling or vibration of one of the wires; most likely the negative line coming off the amplifier to the chassis.  

Good grounds must be solid, tight metal to metal.

Hope this clears it up.  Let me know if you need more.
Cheers,
C