Quote Originally Posted by deafgoose View Post
I replaced my stock ZX200.4 with a ZXM700.5 and after installation I tested everything and it was perfect. That was until I started the engine!

I am getting a really high pitched sound coming our of my speakers while playing music with the engine on.

With the engine off, its perfect.

Any idea what would cause this problem?
I had the same issue. My HU ground was connected to the starting battery neg and my amp's ground was connected to my stereo battery neg. I moved the HU ground to the the stereo batt ground and the noise went away. So check that every component in your stereo system is connected to the same battery, both pos and neg, and the noise should go away.