Earlier today, I was IMing with a friend on my BlackBerry's MSN. I have the tone set to play when I receive an IM as Notifier_Dimples. I received an IM from my friend and heard Dimples, but immediately after, I heard a soft sound of what I can only describe as three piano keys being played.
I immediately checked my inbox; nothing there. I went through the custom sound profile I'd created and tested each alert. None matched the sound I'd heard.
It also wasn't the low battery alarm, as my battery was near a full charge at the time.
It's not really a problem, just more of a mystery to me.