iMac makes strange ticking / clicking noises when intensive CPU usage

Hello.

I own an iMac late2013 27inch.

Recently I discovered, that heavy usage of the CPU will inflict strange ticking noises that come from behind my iMac…

It sounds like little electrical shocks…

It starts when my Video editing software is rendering some heavy video effects or when I convert a BluRay with Pavtube ByteCopy (this will take almost 90% CPU usage)

Then FIRST the ticking noises start (and they get kinda loud) then my fan gets louder to cool the system under heavy usage.

I can definitely say that its not the HDD (because I have a SSD built in)

and it is also not the Fan rattling or colliding with a cable (because I installed FanControl and turned up the speed and no such noises appear when the Fan is blowing 3000RPM)

This is really concerning me as I need the iMac for important Video work right now and cant allow it to blow up in my face… also there are no Genius Appointments left for this week in the nearest Apple store 😟

Did anybody experience this too? Or read about it? I dont find anything on the internet (mostly dying HDDs)

Greetings