Probably a HDD or a Fan.
I would check your essential fans to make sure they are operating accordingly. CPU/GPU/PSU fan failure is not a good thing and could/will lead to component failure.
Case fans are a less immediate concern, but depending on how many you have, it could be a bad thing if it stops working.
If it was your HDD, I would backup anything you can't stand to loose, because this could be a sign of component failure.