PC hardware question - No LBC

Hi List,

The little fan that is fixed to the heat sink on the CPU of my PC sounds like 
it is running a bearing.  As far as I know (someone built the PC for me) this 
little fan is only about a couple of years or so old.  How long ought this 
little fan last?  Also it runs just fine when I remove the PC cover and the 
noise it makes dies down after it has been running a while.  What should I 
do?  Should I just replace this fan?

Daniel1312