In every case the notebook has been motionless. When I first heard the issue it was sitting on my lap, but motionless nonetheless. When I wrote the email, just after hearing it a bunch of times, it had been sitting on my desk.

I haven't heard the noise since. It definitely didn't sound like an HD head movement as it was clearly a beep/blip. Two quick sounds actually. It could have been a sound produced by a piezo disc, but it was definitely a beep.

I did some Google searching and no one was able to explain the beeping sounds they were hearing last year. An update from Apple last fall supposedly addressed an HD issue which some were also blaming the sounds on. But I don't know if these are the same sounds I heard - the samples I saw linked were offline.