Could be interfearence from some other electrical (or wireless) device.
Check to make sure your output mixer has the Microphone mixer muted. There's an input channel called What U Hear which can cause whatever sounds or noises around or even just a fuzz or hiss to come through.
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thanks all who replied...
Can you describe the noise? High pitched? Or maybe the kind of noise you get when you unplg the speaker jack when the speakers are still on-kinda noise?