Most probably a fan. They can gum up with dust etc which sticks to the blades and causes imbalance. And sometimes they need a bit of lubrication but TBH if that's the case are better replaced. Remove the lid and start the computer and listen to pinpoint the place where the noise is coming from.
If it is a fan, turn off the computer, switch it off at the wall but
don't unplug the electric lead. You need to leave the lead connected so that you can discharge any static in your body to earth through it. Touch any bare metal on the computer case and you are ready to start. Don't touch anything inside the computer that you don't need to by the way
Clean the fan(s) blades with a cotton bud soaked in water (but not dripping water). It might be best to remove them to do this but if you do, take a note of which way they face. There are fans on the cpu, power supply, graphics card, case and maybe more depending on the system you have.
If it's definitely not a fan, then it could be a failing hard drive.
Malky