Why is there a buzzing sound in the fuel lines of my truck?

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Why is there a buzzing sound in the fuel lines of my truck? What I have found to date: - When alternator is unplugged, the sound disappears (brand new alternator) - When fuel pump relay is unplugged while running, pump continues to run but vibration changes pitch (relay gets warm when inserted) - Fuel gauge has started acting funny (starts low and works its way to where it should be) - Only seems to be bad on IDLE - runs normal - confirmed on stethoscope sound is only in fuel lines, predominately the return line Any help would be greatly appreciated! I am very confused.