This problem is related to your PC's recording and playback settings. To fix this problem, make sure that you mute the Microphone for playback and select the Microphone for recording. This will eliminate the knocking sound.
To do so, follow these steps:
In the task bar, double click on the `Volume' notification icon
In the `Master Volume' dialog, point to the 'Options' menu and click 'Properties'
Under Adjust volume for', select Playback'
From Show the following volume controls', check Microphone'
Click OK
Under 'Microphone', check the 'Mute' option
Again, point to the 'Options' menu and click 'Properties'
Under Adjust volume for', select Recording'
From Show the following volume controls', check Microphone'
Click OK
Under 'Microphone', check the 'Select' option
