Blue, if your daughter hears it, then others can...so the only other thing I could suggest is perhaps calling your local constabulary to come check out "a baffling disturbance" in your home.

I do know the 'groaning' sound you're talking about, although I don't know where I've heard it before.

Still, you can't go wrong with a couple of cops coming along to hear it as well...if nothing else (barring knocking down walls to eliminate foul play of the human variety), then at least you'll know you're not going mad.

At worst, it's something 'ghostly'. At best, it's a critter with a bad case of laryngitis O.o

Hugs
MorningMist