If there is a mixer (software) that belongs to your sound card you should check that all the ins and out you don't use for the recording are muted. They may generate noise. Also, make sure you mute the other sound tracks in Vegas so you don't get them in on your voiceover track.
Tor

There is an excellent series of short lessons and tips regarding recording with mics in the new Vegas text here:

http://www.sonicfoundry.com/products/showproduct.asp?PID=684

DGrob

Thank you all for your comments...but nothing works...this is driving me nuts. I can record perfectly using Videofactory2 but with all the same connections & settings, in Vegas as soon as I hit record all I get is a very loud, pulsating sound like a machine gun. It is like a vibratto effect. I can capture video thru firewire, import video clips using explorer, and it plays back fine. but when I try to record audio, even with the line-in cable unplugged from the computer, something is generating a loud noise that gets recorded. The only way to stop it is to hit mute.

I've had a problem similar to this and I thought I'd try to help you out. I'm not sure what sound card or driver you are using. (I have "creative") I had to open the record controller and change the input from "what you hear" (what's playing out the speakers) to "microphone" I doubt it will be exactly the same for you but I think this is the problem. Find your recorder and change the setting to microphone. There might be a way to do this in vegas but i don't know it. I hope this helps
Elias

You are getting a "motorboat" feedback. You need to mute something; maybe the line output.

Are you arming the track and/or following the procedure as in the help files?

Might want to try recording in another app, just as a test, to make sure everything is hooked up properly, that your soundcard's mixer settings are set up properly - the windows built in sound recorder will work for this step.

Isn't that much in Vegas to cause prob. but might try different options under the VV4 preferences -> audio device setup.

As you're hearing a lot of noise now, when you're done recording might not be a bad idea to do some noise filtering (Soundforge and the noise plugin works well) as this has to come from somewhere - might be there underneath your recording.

Another thing that I always do for voiceover work is to just plug your mic into a Mini DV camera and record your voicework that way. I find that I get a lot less noise and have a lot less problems if I first record the audio to Mini DV tape then capture it through Vegas. I have my Mic going into a voice processor, then into a mixer and finally into the camera.