First you need to determine the cause of the additional noise in the speakers. Often it may be a bad shielding of the amplifier and/or cable. To check this, take the cable in your hand. If after this the noise became louder, just wrap the cable with foil, or replace it with a new shielded. Regarding the latter, it is the best option, since no magnetic field will not interfere with the sound from the speakers, and the sound will be cleaner.
If the problem lies in the amplifier, the situation is more complicated. Need to disassemble the column and shielded them from the inside with a metal foil. Another option – put the subwoofer or speakers on the processor.
The noise in the speakers may occur due to the lack of grounding. Cheap housing often miss the electrical signals that cause interference throughout the system. To solve this problem, it is recommended to ground the computer. The simplest option is to connect the body, for example, to the battery. Thus, tension in the body will be removed, and the noise will disappear.
Try setting your system mixing speakers. To do this go to "start menu - "control Panel - "Sound . In the tab "Playback find your speakers, click the right mouse button and from the appeared menu select "Properties . Then in the opened window, locate the tab "Levels and disable the "line input .
If all the above ways doesn't resolve the problem, there is only one to buy new speakers.