Your problem can easily be addressed referring to the computer behavior.
If you do not face any undesired halt or freeze and the computing speed is, less or more, that you are accustomed to, the problem is the fan, to be substituted.
In order to be more confident simply launch a detailed HD check or some defragmenting program.
You will be able both to understand more precisely the jerky sound origin and to find vaste amount of errors in the disk in case of hd failure.
If many many errors are not found, change the fan.