Home Studio Soundproofing

I am making a movie with a DV camera, but no external microphone. There is a constant buzzing because the on board mic picks up the motor humming. I am looking for software that can reduce/remove the background noise while not affecting the other audio. Anyone know of any good noise reduction software?

The noise appears to be constant so the noise reduction function in Audacity should help..
Any noise reduction applied will have some effect on the other content with this or any other program.
If the buzz is not too loud the effect should be minimal.
Just left click, hold and drag to select 4 - 5 seconds of just the buzz.
Go to the effect menu and select noise removal.
Get a profile then select remove noise.
The sound track will need to be imported into Audacity as a separate sound file in wav, mp3, Ogg or aiff format.

Any manipulation of a sound file will affect the final result so the best solution is to use an external mic to eliminate the buzz.
The inbuilt mic on a video camera is for convenience not high quality.
Even a cheap mic will be a big advantage.
The aim of any recording is to get the best possible result at the time of recording.
In this case the removal of the buzz might have minimal effect but if this is not the case and the removal process is too disruptive the only solution will be to purchase an external mic.

Audacity:
http://audacity.sourceforge.net/
Lame if needed to encode to mp3:
http://lame.sourceforge.net/
How to install Lame:
http://audacity.sourceforge.net/help/faq?s=install&i=lame-mp3
Basic tutorials:
http://audacity.sourceforge.net/manual-1.2/tutorials.html
http://www.recipester.org/Software/Audacity