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:
1. In the task bar, double click on the `Volume' notification icon
2. In the `Master Volume' dialog, point to the 'Options' menu and click 'Properties'
3. Under `Adjust volume for', select `Playback'
4. From `Show the following volume controls', check `Microphone'
5. Click OK
6. Under 'Microphone', check the 'Mute' option
7. Again, point to the 'Options' menu and click 'Properties'
8. Under `Adjust volume for', select `Recording'
9. From `Show the following volume controls', check `Microphone'
10. Click OK
11. Under 'Microphone', check the 'Select' option