So last night during the shutdown sequence (i.e., after I initiated shutdown and before the computer powered down) my early 2008 Mac Pro (running Lion on a Vertex 2 SSD) made a prolonged squeal that sounded literally like a smoke detector going off. It was long, high pitched and VERY LOUD. In the 15 years I've been using Macs I've never heard this sound. I had two external SATA enclosures running which I had powered off around the same time so it's possible it could have been one of those. It didn't sound like a fan motor tweaking out, it was like an "intentional" sound if that makes sense. The windup... everything seems normal. After a few seconds, the shutdown completed and when I rebooted everything was fine and it hasn't happened since. Any clues?