audio recording on sensing level similar to motion detect
 

I have tried google and searching the forums, but I'm no
t even sure what its called. I'm looking for an application that will record audio as long as the level is sufficient. The best analogy I have is motion detection for video. I want to setup a microphone to record while I'm out of my office. There's been some problems with a few machines spinning fans at full speed. The logs don't help and I'd like to know how often it happens. I have about a dozen machines in my office so normal background noise is high. I would need to adjust the recording threshold to match. Is there an application that does this or a way to monitor the level and trigger a recording start when the threshold is breached and stop when the level drops for x seconds?
Thanks for any help.
Traigo