quacking noise when network cable disconnected or wakes from sleep

My macbook pro makes a very odd noise when waking from sleep.

It makes a sort of 'quacking noise' Sometimes it quacks once, sometimes multiple times.

having experimented, I have discovered that this noise also occurs when I take the ethernet cable out of the side, so I assume it is some sort of 'network disconnection' warning noise.

I can't find a way to stop my mac doing this.

Any ideas?

cheers
Dom