Why is my G5 honking at me???!!

Hi - my G5 is working fine without any problems, but has been making some strange tones for no apparent reason. The noise sounds like a cross between a goose and a car horn and is a fairly short blast. It 'honked' once when I tried to reply to a message whilst in Safari and then did a series of 3 or 4 honks when opening Adobe inDesign, but no error messages appeared and everything just continued as normal. It's really puzzling me, I don't have any webpages or other applications etc. open that could be making strange noises in the background and it definitely came from inside the CPU!

I've backed up my recent projects just in case it's about to conk out, but does anyone know what's happening here??