It's the save summer/eat ice cream burgers one. One of the adchoices? Really bizarre as it's not a video and doesn't move but the sound is only playing when this ad is on my screen.
It doesn't start straight away but after a few minutes of me being on a page, took me ages to work out where it was coming from 
it's when I've scrolled right down the page too so I'm not hovering on it by accident.