Try with the camera on battery instead of ac. If this doesn't work try testing your capture card with a different audio source such as a portable cd player to see if the sound is comming from the card. Frenchy is probably right though, because it does sound like a ground loop. Hopefully isolating your camera from outside AC should do the trick. Also make sure to have only one audio connection hooked up at a time.
Steve S.

I'll try the battery instead of AC, sounds like a possibility...
Thank you

I'd like to thank everybody who answered my question, I appreciate it very much!

Well, my camera was connected via pretty busy outlet, so I tried a battery, as advised. However the real solution in my case was to bring down (or mute) Bass lever on the Windows Master Volume panel.

Regards,

novice