all forms of engine noise and hiss are caused by poor grounds. If you have a clean ground in a good spot.. there will be no noise travelling up the lines. If you have poor grounds, then you have noise. You can run the absolute shittiest RCA's next to the biggest powercable you want, and as long as your equipment is grounded PROPERLY, then you will have NO problems.
For the people who have had an issue with their system.. their ONE system. Get a clue, most of us do this for a living, we wouldn't be saying this stuff if it wasn't true. We do this all day long on multiple cars.