I would like to add some info. First, I have five computers making this same noise and all five just happen to be running Windows XP. Further, all five went through a procedure of software additions, namely the installation of Symantec DLP/antivirus software and were joined to a domain and installation of Microsoft SCCM. After that, this noise started happening on all five computers. There may even more of them I just haven't discovered them yet.
Not all the drives are Desktars. At least one is a Seagate.