Why does my Mac go "whoosh" sporadically during the day?

Occasionally my MacBook Pro makes the "whoosh" sound. It is the same one you you hear when dragging an icon off the dock and it goes "poof".

I never notice anything different.

What is it doing?

Do you use iChat or Messages? The same noise is used to indicate one of your buddies has signed out.

the swoosh sounds can be found by right clicking on ichat in the application folder and selecting show package content then go into contents then resources and its "buddy logging in.aiff" and "buddy logging out.aiff"

Do you use apple mail?

If so, it could be an email being sent and playing the sent email sound.