I've been using Macs since 2007 and I've never heard this sound. Sometimes (but not always) when I open my newish MacBook Air, after around 15 seconds, it makes a dinging noise, kinda like a Melbourne tram (lol). I'm not really sure what's causing it. I thought it might be InsomniaX, but its sound effects are off. I'm totally stumped. It's not really a problem, it's just... weird. Any ideas? Thanks 
