Every so often, noise will start playing if I'm on a mumsnet page. I have other tabs open (Google Chrome), but it is Mumsnet with the little loudspeaker icon, so I know it's coming from Mumsnet. It sounds like one of the adverts where the embedded video clip starts playing automatically, but there are no moving adverts at the time (and no adverts that seem to relate to it).
It's quite faint, and hard to make out. It's weird.