2011 iMac big noise in back

[lidalee]

My 2011 iMac have big noise in bottom back and power button is not sensitive
somebody knows where hardware is wrong??

this is video url :

thanks

 

[Fishrrman]

Sounds like a fan or perhaps a platter-based hard drive that is getting older (and noisier as it ages).

 

[Princess Cake]

Sounds like the fan to me, might be some crud in it or maybe it needs lubricant.

I'd start with a can of airduster and blow the heck out of the bottom, center, and top air vents.