1. The rushing noise (hiss) is probably produced by the stock head unit and amplified by the amp. There's nothing that you can do about it besides changing the HU
You can try to confirm this by unplugging the high level inputs on your amp, plugging in an mp3 to the amp via RCA to 3.5mm
If the noise dissapears, this confirms that it's the head unit causing it.
2. Alternator whine
Many possible causes. Since your power and REM cables are separated from the speaker wire, it shouldn't be the cause of the issue.
Look for exposed wire. If you have bare wire touching the vehicle body, it's certain to introduce the whine. If possible, check whether the whine is coming from any particular speaker.
If it's coming from a specific one, you'll want to check the wiring for that speaker. Even a strand of copper touching the vehicle body WILL introduce significant noise.
It's quite unlikely that a poor ground is the cause.
I'm running 2 760w amps and I'm not getting any issues despite simply bolting the ground cables to existing holes in the vehicle body without sanding.
Source of knowledge:
many, many, many hours of climbing over my car like a monkey