That can be anything from dust, to driver, to connection to interference. For 2 channel use the (green) stereo jack. You can try to clean with a degreaser like Isopropyl alcohol. Dampen the jack with alcohol twist around in the plug, clean off and repeat. Failure try an updated drive and try a higher sampling rate in the control panel.
Latest drivers - http://www.realtek.com.tw/downloads/