Most likely from the optical drive, did you push in a disk while it was off or asleep?
It is a mid 2009 model. I think the sound came from the area between the display and the unibody.
I opened it while it was asleep.
Something broken in the hinge perhaps? try opening it up a couple more times?
Opens normally.
You are sure it's not a System alert or an Apps notification sound, a "bing" sound is kinda strange to be hardware related, but then again it's hard to define a sound, without a recording of the sound it's hard for other people to imagine.
It sounded like an exploding light bulb.