Jay says his wife's computer will make the "swoosh" email sound frequently when there's no email being sent or received. Leo says to make sure there's no sent items in the email sent folder. Also, check in the settings to see if that sound is being played for other things the computer does. There's an option called "play sounds for other mail actions." Make sure that's disabled. Also, Leo advises using a program called "Little Snitch" for the Mac that will advise him if any nefarous activity is making an outbound connection, but it's probably not that.