If I fiddle with it a bit it occasionally goes quiet. But the vast majority of the time there is this loud buzzing in the background. Any idea how to fix it? It's not my pin connector or anything, because I just replaced that.
You would do a lot better NOT using an acronym, and if it stands for Nintendo Entertainment System, you would do better posting in the gaming forums.
(1) Try a different outlet, you might have something connected to the outlet(could be somewhere else in the house on the same circuit)that is causing static on the circuit (the noise). (2) Try cleaning the cartridges contacts with a cue-tip and rubbing alcohol. (3) Try a different av cable, the one you are using might be damaged (4) try cleaning the av cables connectors to make sure they are not dirty. (5) check the where the pin connector are connected the board to make sure none of the points are bridged by mistake.