How can I reduce noise from my signal?
 

So I installed the 4 channel x 45watt Alpine ktp-445a on my Brz a while back. I am powering the front door speakers at 45 watts.
Gain is turned about halfway so door speakers a louder than the dash speakers(sounds harsh/glaring).
With the volume at 0 on the head unit, it sounds like a fan is on. It is bothering me quite a bit.
I am using the factory wired ground.

How can I reduce the noise?
Should I make a chassis ground? Should I keep the wired ground connected also to have 2x the ground?
Filters?

Also, how do I turn off the head unit completely? I can turn off the screen, or turn off the input, but the head unit is still on and the noise is still there. Ideas?

Speaking from personal experience with the KTP, you can try making a chassis ground, you can also try making some sort of ground that attaches to the head unit somehow, that attaches to the same chassis ground you use for the amp. Make sure all bare wires are totally insulated or covered, that they don't touch the chassis at all, even the amp inputs.

I commented previously based on skimming what you wrote. Definitely not the cause when I read through it.

bump for more ideas or how to turn off the head unit completely.

Not usually a problem for lower watts but if you are using RCA cables and they are near the power cord it may cause interference. Just something else to look at if the ground doesn't fix it.