Mumbling again

I had a short period of mumbling night before last. It occurred just before midnight and lasted between five and ten minutes. The voices, a least two of them, were female, just low enough that no words could be made out. It was downright cold outside and spitting ice crystals. I first noticed the mumbling as I exited the Library into the dining room. I walked out from the dining room into the parlor thinking it was someone outside. There was no one visible outside and even kids playing in the street in the daytime do not carry in through these thick walls. I retraced my steps back through the dining room and into the kitchen where the mumbling seemed about the same. I was about to go upstairs where I would have a view of most of the surrounding territory (as much as darkness would permit) when the mumbling faded away. I have pretty much discounted that the voices were coming from the outside. The walls of this house are thick and the storm windows are triple and were closed (the temperature was around 24 degrees). We have never figured out the times we have had actually loud, easily understood voices. The mumbling is far more complicated and an even greater mystery. The loud voices originate from a completely closed off dumbwaiter that use to service this house. The loud voices are very rare but when they occur they are very clear. The mumbling, on the other hand, is consistant though out the house but never loud enough to understand. Considering the time of night and weather conditions on this last episode I think I can rule out people outside. But what in heck was it?