Engine hums when below 1000 RPM, code P0174

Recently we had to go on a couple long trips, each 600+ miles round trip. On the second trip, we noticed that whenever we slowed down and the rpm's dropped sharply or dropped below 1,000 rpm's, the passenger side of the engine has a loud hum. It almost sounds like a conch shell being blown or a note on a low brass instrument. If we pop it into neutral and keep the rpm's up, it doesn't do it. It also tends to do it less when the engine is cool and/or we keep it below 15 mph. The check engine light is on, it has a code of P0174. We cleaned the MAF, just to be sure, and also all the fluids are good. Does anyone have any ideas? Its really loud and obnoxious, and in a couple weeks we're going to be doing an 800 mile round trip, and we don't want to break our only car worse. Thanks everyone!