Does it occur whenever it is powered on, or do you think it is associated with the cooling fan?
The fan may simply have ingested a piece of debris. It can be cleaned, but that task is best left to Apple or an authorised service shop.
If the noise is present all the time it is running, it could be signs of impending hard disk failure. That can also be repaired. Be sure to back up all the data you don't want to lose.
There are no other moving parts inside a Mac that could explain this. Try running Apple Hardware Test. Click to learn how: