That's interesting...I get the same thing myself from time to time....hearing low voices or whispers, but I can never make out any words...it mostly sounds like there's a radio or something on, but turned down just low enough that you can't make it out! I hear it more at night when it's totally silent. It doesn't frighten me, it mostly frustrates me because I can't pinpoint where it comes from or what they're saying!
There are many times when I'll be sitting somewhere and just start turning my head like an antenna, trying to figure out what direction it's coming from. People around me will ask what I'm doing and, when I tell them I'm hearing something, they always respond that they don't hear anything....frustrating, but I'm kind of used to it by now.