From what I am guessing you are saying on the sub amp.... When you go to turn the car off and the head unit is still on....you here a short pop. On my sub, I had the same prob. with when I turned it off I got a quick thump, or pop. The reason for MINE to do that is becuase my sub amp Capacitors were releasing (draining there power) everytime it was turned off. I dont know if that is the samething as what you are saying yours is doing. I dont think there was a fix but I will ask some guys here in the shop, about that.
Check your ground on your amp for the highs....for ground noise.
The ground noise you are talking about is high freq correct? YOu should only here it when the car is running
I just dont want you to get it confused with Pink Noise.
Good Luck,