One random change I noticed during the F18 cycle was that the default notification noise changed from an innocuous 'bell' sound to a 'water drip' noise. While I turn off all terminal noises, this sound still pops up in Firefox and Thunderbird when a ctrl-f quick search doesn't find a match, and in my F18 VMs with default configuration. And while I'm sure many developers just turn off all sound effects, I find them useful for email and IRC notifications.
This new sound is offensive to my ears, one small step below nails on a chalk board. Unfortunately there is no way to undo this change in gnome-control-center's Sound panel, just 2 choices for the drip noise ('default' and 'drip'), and some other comical choices like a dog 'bark'. Nothing as unoffensive as the previous default. I (just yesterday) filed a bug against control-center asking for a way to restore the old behavior.
However, if you want to revert it in a hacky way, this should cover you until the next time sound-theme-freedesktop is updated:
sudo wget https://cgit.freedesktop.org/sound-theme-freedesktop/plain/stereo/bell.oga?id=38bc773912317a2163083b6f483fbc8e6fb61123 -O /usr/share/sounds/freedesktop/stereo/bell.oga