You might want to run the Apple Hardware Test to see if it reports any hardware failure. If you get no error reports form the AHT, I would take the machine into your local Apple Store for better hardware diagnostics.
The Apple Hardware Test (Extended Version) found no problems.
I'm going to book an appointment with the Genius Bar. I hope they can find the cause.
I did buy a new MagSafe power adapter recently.. The one that came with my computer was a 60W. The rep who sold me the adapter advised me to get the 85W this time. He said the mid-2009 model could use one or the other, but that the 85W would be more "forward compatible."
Now I'm wondering whether any possible defect with the adapter might have anything to do with the noise the computer is making...
The visit to the Genius Bar did not shed any light on this issue.
The rep did not do any hardware testing, since, according to him, if the problem was hardware-related, there would typically be more than a single random beep.
He suggested updating all of my software and keeping an eye on console messages.
I have been doing that, but there are no particular console messages being displayed when the beep occurs.
I have noticed that if I lower the speaker volume, when the beep does occur, it is less audible, i.e. the volume is dependent on the speaker setting.
It's happening 2 or 3 times a day now. Not really a problem, but I would like to know if there's anything that needs fixing inside the machine.
Has anyone else had this happening with their MacBook Pro?