Most likely the bearings on the cooling fan(s).
Open the case up and make a "stethoscope" from rolled up paper and you should be able to see which fan is causing the problem.
It isn't worth trying to oil the offending fan, they are very cheap to buy new. If it is the heatsink fan, make sure you get on to match the heatsink you are using. Case fans are fairly standard, provided you get one the right size. Fitting should be obvious but ensure the direction of airflow is correct and matches the one already installed. There is an arrow pointer on all fans to indicate the direction of flow.
It could, of course, be the hard drive that is making the noise. In this case, make a full backup of your system immediately as it may happen (as it has done to me in the past) that when you shutdown, that is the last time to drive will run.