Upgrading the size of your power/earth cables generally wont get rid of "buzzing" or engine interference (unless your using like 16awg cable). As a minimum you want to use 8awg power/earth cable.
Normally this is caused by a poor earth connection. Make sure you sand/scrape back the area to bare metal, and as lil_mickey said earth it to a solid part of the car. The rear seat belt bolt is a good spot.