The power inverter is the culprit. The power that comes out of it is a modified square sine. If its not a high end inverter it will create a lot of interference. Not only does this create annoying buzz sounds, but this is actually very hard on the equipment after a long period of time. This is just my opinion on the issue, I cant be sure without assessing the problem but, If you want to kill the buzz sound look into these:
Install this between the PC audio out put and the head unit input. This should do the trick, good luck!