Audio Systems: car audio system, cheap amps, car subwoofer

Question
i had an audio system installed in my car (subwoofer plus amp) at first the car people said everything is fine but when i turn the key on the sub's make a pop and same happens but 3 times when i turn the key off, when the car is running i can here engine noises from the amp or subs its coming from the back im not sure whats going on.
2004 for mustang the stero system or playing is stock (mach) system

Answer
as for the popping, some cheap amps just do this and there's not a whole lot you can do about that.  the engine noise is a different story but can be hard to find and fix.  
I think the way I'd start is to disconnect the RCAs going into the amp.  see if the pops or engine noise still occurs (of course the subs wont make music this way).  if the amp still pops without the rcas then it's the amp doing it by itself.  if the popping stops, then it's being "fed" the pop from the cd player or somewhere else.
same thing with the engine noise.
try that, drive around, turn the car on and off etc. and let me know.