Can somebody please help me?

I am capturing video from my analog camcorder using Studio PC TV Pro capture card. I am using Virtual Dub for capturing the avi file. The captured video is very good in quality but the audio part is not clear.

At the time of preview of both video and audio are clear. As soon as I start capturing the video a hissing sound with some noise. So although I am getting my original audio, it's not clear.

This happens if I am capturing using Virtual dub, Studio software or any other video capture also.

Appreciate your help on this.