I have just begun using acid 3.0 and vegas LE. Anytime record is pressed there is a low level hum/buzz, that ruins any recording. The buzz happens with and without any line input into the mic jack on my computer.
I used to get a similar problem with recording software. The problem was caused by a noisy output from my CD-ROM that was being mixed in to any signal via the SBLive mixer. Once I disabled the CD input in the mixer, the problem disappeared.
You don't say what soundcard you are using, but could this be the problem?