iMac rattling sound

Hello people!

I really hope someone can help me out here.

My imac is rattling! I've had a loud fan noise coming from my imac before, but back then it was my hdd.

That was, after crashing and dying on me, replaced.

Now i have a same sort of noise coming out of my imac. This time i opened it again, thinking it was my hdd or something.

It was not, it's actually the fan thats behind the stand of the imac. I tried googling this but no one mentions it.

If you open up your imac, it's very hard to reach this fan, its behind the logic board(?) and almost all other components.

Here is an image:

It seems to be from there. It's (usually) extra loud when i'm rebooting or waking it up from sleeping mode.

It's less noisy under windows in bootcamp (odd?). In osx you need to play loud music to cancel out the rattling.

Still its varries in loudness but it does not seem to relate to intensive use or no use at all. Quite random.

Any tips? Really hope someone had this before and knows a way to fix it. It destroys my serene imac experience, especially cause i've been running windows lately.. Blasphemy😮!!

I recored the sound but it does not let me add it to the post.I'll look into that if someone wants to hear it.

Thanks in advance!

Greets