Sometimes you will hear soft hissing or other noise when no music is played. If the speaker does this while no leads are connected (and preferably without any wireless gear in the vicinity like bluetooth and wifi on tablets and phones), the problem most likely sits within the speakers themselves. And most likely involves the amp or cross-over.
If it only makes the sound when a cable is connected (what cable are you using?), then it can be a host of other things. Even the USB cable can be the culprit. You should, if at all possible, use the XLR outputs and XLR inputs on the speakers to eliminate problem on the speaker cabling.
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