Depends on the heat settings, my friend!
If the temperature of the CPU goes too high, the fan will kick on to cool it off.
However, if it drops below a certain point it will shut off again.
Sometimes browsing can be simple, sometimes it can be very demanding. (Like if you're scrolling through a big news feed on facebook or watching a youtube video.)
It's meant to do this to keep noise levels down. Which is apparently the new thing that people are worried about.
My old PC sounded like a jet taking off, but it ran cold. lol