When you say high-pitched, what frequencies are you talking about? If you need to compare, there is a frequency generator in Room EQ Wizard, available for free on this site. You can listen to some tones and tell exactly what frequency the hum is at. That may help diagnose the problem.
When I hear hum, the first thing I check is the cable connection. This is usually for the lower 60 Hz hum from the power line ground loop. Best way to check is to disconnect the cable from the wall to the set top box, power strip, VCR, TV, etc. If the hum disappears, that's your culprit.
If the hum remains, try looking for any piece of gear that has a third prong, like an amplifier (although it could be anything). Unplug that and see if the hum remains (as long as it isn't the one thing powering the speakers
Higher pitched hums can be radio interference on some of the signal wires, try moving them around to see if it goes away. A bad capacitor in a crossover can cause this as well, but that is much less common.
Let us know what you find and we can help with a solution.