hearing voices...

I've been recently spending more time alone in the building where I work. Although I'm not as creeped out as before, I've been hearing things that sound like voices coming from empty offices and rooms surrounding my office. At first I thought it might be the heat or sound coming from water or heat pipes... but I found that the place I thought the sound was coming from had been moved out and emptied..several months ago. I wish I could understand what's being said, but every time I get close, the voices stop. I'll have to set up equipment sometime soon!