I realize this tool
can be extremely expensive and may be a technical nightmare to use. If you have the technical expertise and money to afford such a tool it can be placed up against your ear and the other end can be placed close to suspect components. If you are careful fans can be stopped momentarily with an eraser or similar smallish item that won't break the blades. Just be careful. Some fans can cut human skin with ease. I'm thinking Scythe fans. I suggest that to rule them out as the cause. I was sorta expecting you to find a cord to close to a fan blade like Mark suggested if it weren't your HDD.
https://www.hwinfo.com/ is a program I'd download. It will show you your drives and will have a circle or something to the left of the drive. Red, yellow or green it will be. If it's yellow there's a problem. It can show you if any of your drives have issues. Many hard drive manufacturers include or offer a hard drive testing utility.