Which fan is making the noise ? A case fan , cpu fan or gpu fan ? What are sys specs ?
According to the manual to my computer it is the assembly fan #T4307 Fan and Shroud It is covered with a green shield.
It is a Dell Dimension 8400 P4, 630 (3GHz), HT, 2MB, EM64T
1GB DDR2 SDRAM
250GB SERIAL ATA, 7200