Its alternator whine man, and all of that stuff could be a possibility. Make sure the paint where you have it grounded is sanded down to bare metal. If possible take a multimeter and measure resistance between the location you're grounding and the negative battery terminal. make sure everything in your system is grounded at the same location...not 3 feet away from each other.

The box your friend was talking about is a ground loop isolater, they work like 50% of the time so you could try that but it will cost you a little money...usually like 10-15 bucks.

listen real close to the sub and flip it to all pass for a couple seconds, sometimes the LP crossover will filter out the whine so you wont hear it as predominately as the highs amp. You can also check if you have bad RCAs by just getting a set off of a friend and tossing them from behind the deck back to the amp, not having to run them under the carpet, because as said...theres so much stuff it could be...I've even seen bad spark plug wires cause the issue in a couple instances