Cford has it right. Be certain that the black wire(hot) is connected to the copper screw of the outlet, and the white wire(neutral) to the nickel screw. You might even check the breaker box connections, while turning off the power, as Dave suggested. You should only see black wires connected to the breakers, and whites to the neutral buss(could be reversed there instead of at the outlet).
House Wiring Defect?
Recently I introduced a new power amp connected to my preamp with XLR cables. The pre and power amps were plugged into different outlets in the same room which produced a pretty loud hum from both speakers with the preamp on or off and the power amp on. After the inital freak out and a check of all connections I realized one fix was to power the pre and power amps from the same outlet. This restored quiet operation using either outlet. The question is, does this indicate a house wiring defect or nothing to worry about?