Many people worry when their computers seem to hum when a request is made to open a new program or to boot the computer. In reality, this whirring hum does not mean the computer is in serious danger of experiencing a crash, fatal or otherwise. Part of the sound is based on the actual movement of the arm and platter movements in order to get the requested file or function up and available and part is due to the placement of those files on the platter since some of the information needed may not be stored in the same sector on the platter, thus causing a need to move the arm and platters more in order to locate and retrieve said information.
Even though the speed at which a hard drive works is slow when compared to the speeds of other technological computer functions, the speed at which the arm moves the headers, which read and write data on the platters, is impressive. Able to move hundreds of times per second over the face of the platters, the arm is a precise and efficient method of data retrieval for files that have been requested.
All of this moving, along with the motor that spins the platters, creates vibrations which are then heard as a soft whirring sound during the hard disk's active periods. It is really no different than the basic concept of a stereo speaker, which works by moving a lightweight cone back and forth hundreds of times per second to produce sound. When placed in this perspective, the whirring you hear is a sign of health for your computer as it is evidence that the computer is working and retrieving requested data.
What causes all this movement and why is so much needed that an audible sound can be heard? Well, when you click on an application's icon to retrieve its data and begin use of the application, several things happen. It doesn't just open up all at once, although it may appear to your eye as if it did. The hard disk has to load the application along with several dynamic link libraries, or DLLs. The sizes of these files differ and might contain 20 megabytes of information. Now add in that more motion is needed by the arm and the platters if the information is scattered on the platters instead of neatly organized due to the lack of defragmentation of the disk.
After all of this, the data file that was originally requested has to load. The operating system (OS) lends a hand by moving the head to the drive's directory, verifying the existence of the file name, and finally locating the file. The OS further assists by reading the dozen or more tracks, which may be scattered all over the platter, to access the file.
Next, your computer's OS does a feat that is amazing. It checks the RAM available to the application at the time. If it is full, the OS simultaneously tries to write the bytes of data, which may number in the millions, to the paging file on the disk in order to make room for the new application while it loads the application in bytes along with the DLLs necessary for the application to urn in order to allow the program to be accessed in pieces to save RAM.
You, of course, never see any of this, but your computer does it all. That means the drive head is moving all over the hard disk to accomplish the task you set for it, which produces the sound you hear, while completely keeping you in the dark about how big the job really is. The sound you hear is the only indication that your computer is working overtime.