Odd noise coming from pc

[Estaric]

I woke up this morning and noticed a strange humming coming from my pc that iv never heard before. Id compare it to coil whine but maybe a bit lower pitch. I immediately restarted my pc and the noise would go away in short bursts and then come back during the boot up process. I decided to take everything out reseat all the hardware and make sure the cables were plugged in all the way and now i cant get the noise to come back. Any idea what this could have been and if it should raise any red flags for me?

 

[Regeneration]

Could be a dirty fan, dead spider/mosquito, HDD, electrical interference from a local power surge.

 

[8BitZ80]

Have you disconnected your hard drives to see if that makes it stop?
I had a Seagate Barracuda develop a small vibration and it made annoying humming sounds when it resonated with the case.

 

[Estaric]

I havent been able to get the pc to recreate the noise again sense i reseated everything. I did suspect one of my hard drives but untill i get the noise to come back im not sure what it could be.

 

[trog100]

if its gone leave it at that..

trog

 

[joemama]

it could be vibration of some loose component

 

[Estaric]

Okay so the noise came back. Iv determined that its one of the case fans going bad. I appreciate everyone who gave me some ideas.