It's a sign of impending failure of the hard drive. You better backup ASAP.
Failing Hard Drive Sounds http://datacent.com/hard_drive_sounds.php
Listed by drive manufacturer. Click on the > next to the drive & you will hear the typical failure sound.
How to Tell When Your Hard Drive is Going to Fail
Grinding And Clicking – Symptoms Of A Failing Hard Drive
Failing Hard Drives
Some newer Macs use a technology called S.M.A.R.T. (Self-Monitoring, Analysis, and Reporting Technology) to attempt to detect problems with the hard drive that indicate it is about to fail, such as mechanical wear. If a SMART-equipped (SMARTReporter.app; Mac OS X 10.3.3 or later; at least one ATA, SATA or eSATA hard disk drive) computer detects that its hard drive is about to fail, it can warn you of this so that the data on the disk can be backed up.
You have the only Mac without a hard drive??
Your problem may not be exactly the same as that of the original poster of this old thread, and it can be very confusing for everybody if we try to answer more than one question in each thread.
In order for us to give your problem our undivided attention to try to solve it, would you kindly start your own thread, describing the trouble you are having in the fullest detail, including completing your details to show what Mac you are using, what operating system, and what version of the application in question. Please remember to post in the forum relevant to your hardware or version of OS X.