I am running a Dell PowerEdge 1800 and have been for a few years. All of a sudden, the server is now making a high-pitched buzz and doesn't go away. I have restarted the server, but it eventually comes back. I have also unplugged and turned off the battery backup as that was thought to be the initial source.

It is the alarm for your CERC SATA 1.5/6 RAID controller. Install OpenManage Server Administrator (Managed Node) from Dell's Support Site to see which drive has failed. Test the drive. If you cannot test it using Online Diagnostics or it fails the diagnostics, then replace it with a drive at least as large (can be larger) as the original. You can also boot to CTRL-A during POST to the Configuration Utility for the CERC, but ideally you would use OpenManage. It is the alarm for your CERC SATA 1.5/6 RAID controller. Install OpenManage Server Administrator (Managed Node) from Dell's Support Site to see which drive has failed. Test the drive. If you cannot test it using Online Diagnostics or it fails the diagnostics, then replace it with a drive at least as large (can be larger) as the original. You can also boot to CTRL-A during POST to the Configuration Utility for the CERC, but ideally you would use OpenManage.