Rather than pulling apart outlet and breaker boxes, follow Zapper's advice and get a 3 light tester. It looks like a yellow 3 prong adapter, with 3 lights on the back. You plug it into the outlet, and it will tell you if anything is wired wrong.
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?