2011 iMac 27" high fan spin at light use

[Dell16071]

Hello everyone. This is my very first mac so I am new to all this.
I was gifted a 2011 iMac recently. It will only be used for light photo and video editing in Lr and Pr. The only issue I have noticed is after about a few minutes the fan starts to wind up at very low usage. Watching YouTube after about 10 min makes the fan so loud it distracting and it will not wind down even if I turn off YouTube, yet the casing is cool to the touch.

One other thing is this I mac was dropped a while back and it dented the casing and chipped the glass a bit on the top right corner. I didn't mind because it seemed to work fine, but now I am concerned something might have gotten damaged in the fall. I bought a new hybrid HDD for this iMac to speed it up so I will be opening the screen anyways soon. Is there something I should maybe clean or see if something is loose or damaged inside?

iMac 2011
ElCapitan
12gb ram
i5
27" screen

 

[FlorisVN]

what are your imacs temperatures ?
I think there could be a lot of dust, and old thermal paste inside which could make chips run hotter..

you can check your temperatures easy, with for example the free app macsfancontrol
https://www.crystalidea.com/macs-fan-control/download

make a screenshot of them for example, and show us here on this topic.
Do this for example in idle mode, and when you are editing a photo and video for example doing some work loads..