Some SATA drives produce a weird noise when shutting down. This is due to shutdown(8)'s incorrect spindown procedure.
General info: http://linux-ata.org/shutdown.html
Technical info: http://bugzilla.kernel.org/show_bug.cgi?id=7674
This bug is important because the lifetime of our hard disks is getting shorter.
Our shutdown isn't patched to send SATA disks into standby, so this issue does not affect us.