Short noise when phone disconnects from wi-fi

Whenever my phone disconnects from wi-fi, it makes a short noise. Apart from why it is disconnecting all the time, the noise is annoying because the phone is set to vibrate - hence it shouldn't be making any sound at all!

Any ideas on what is going on and how to fix it? I'm using the stock 5.1 ROM on the S6 international version.

Thanks,

Ian