This indicates that the input level is too high. Lower the volume until 
the popping stops. 

The Audio Mixer has advanced option buttons that set the gain for the 
Mic and CD/Line inputs.  Change the amount of gain by clicking the small 
button to the lower left of the slider.  The meters indicate the level of 
signal applied.  If the red lights are on most of the time when audio 
is applied to the card, the signal is too loud.  Reduce the gain until 
the upper red lights light up only occasionally.