Fuel consumption, engine humming and whistling

I'm experiencing excessive gas consumption, 7-8 mph less than normal. What could be contributing? I have been hearing a high-pitched humming or whistling from the engine, I think, especially when I'm accelerating at higher speeds. The check engine light is always on. I think I feel a misfire when it's idling, but I just changed the spark plugs, wires and distributor cap less than 2 years ago. There are also fluctuating idling speeds. I'm not sure if the problems are connected, but what might some of the possible problems be? The diagnostic codes are P0300-P0304, all five of them.