This is a bug where for some unknown reason there starts to play an irritating circus type music, I believe it is one used in the Showtime performances, although I suppose it could be others if you don't have that expansion. It is not the ghost hunter music that sometimes keeps playing all night long in a place it shouldn't, or the computer game music that also sometimes plays at times and places where it shouldn't be. Unlike these, this is tied to one particular family member and follows her wherever she goes. If you zoom in close, the music gets stronger, like she is a walking stereo.
I solved it by sending her to University and back. For those who don't have that expansion, I would guess that the tourist destinations would do, and probably the city of the future. Perhaps you can also solve it by moving to another house, certainly to another neighborhood (but that will wipe all your relationship outside the household). Resetsim and Fixall (from Awesomemod) did not work though.
Posting this in case some unlucky soul doesn't find it out on their own. ^_^