What causes TURN ON/OFF POP and how to cure it:
When a car audio system utilizes additional components such as amplifiers and processors, those devices are turned on and off automatically with a signal from the source unit (known as remote turn-on).
Occasionally, those components may have built in delays that turn them on and off at different intervals. This is especially common when mixing different brands of components into a single system. These different turn on/off intervals may cause an audible "pop" or "thump" when turning the system on or off.
For example, if the amplifier in a system turns on before a crossover, the crossover may send a small transient signal to the amplifier. Because the amplifier is already on, it amplifies the transient signal causing the turn-on noise.
There are different methods to cure a turn-on noise, including devices that "delay" the turn-on of the amplifier until after the other components have come on and "settled down". These delay turn-on devices are available from most car audio accessory suppliers.
To cure a noise at turn-off, the amplifier must shut off before the other components in the system. To do this, you must delay the turn-off of the other components until after the amplifier shuts off.
This sounds like a lot of work to figure out, and it can be frustrating to get rid of such noises, but patience and a little "stick-to-it-iveness" will help you to find the source of your trouble and fix it