This keeps happening to me on MN - I have all ads set to mute but while scrolling past, they start to play. It's really annoying and means I can't read MN late at night (I have insomnia) or I'll wake dh.
The most recent one is one about foods to eat while in labour. It was on this thread:
https://www.mumsnet.com/talk/am_i_being_unreasonable/4884535-reborn-dolls-in-the-pram-bay
There was another one earlier with some piano music.