I found it -- search for all .wav files and looked for the sound itself, the path to the sound then told me what was using it. A sidebar gadget that monitors network traffic upated itself and enabled the sound.
About four weeks ago, my Dell XPS One 27 started making this really annoying klaxon sound every time the network cuts out. Despite my best efforts to keep the card active, this includes when the machine wakes from sleep. Anyone got any ideas as to:a) turn the sound off completely b) change it to something really quiet  c) keep the network card alive all the time (and yes, I unchecked the power saving box already) Thanks Jules
Annoying klaxon sound with network connection