PC making rattling noise

[Deleted member 2621623]

I just built my very first PC yesterday and everything were flowers until I started hearing a somewhat metallic/rattling noise coming from it when the processor needs to work harder. It's very similar to this one https://youtu.be/1ukeJ0u3srI?t=1m52s except that it's way quieter on mine.
I suppose it's coming from the water cooler radiator fan since it's connected onto the cpu fan on the motherboard. But I really don't know how to fix it.
If anybody had this problem before please help me.
Cooler model: CoolerMaster ML120L RGB.

 

[kerberos_20]

if u startup only your water loop (no fans, only pump), does it make same sound?

 

[rhoban]

Could be a loose screw or something. Perhaps even the case.

The top of my case sometimes makes a rattling noise. If I press my hand on it, it stops.

 

[Sandjai]

For me the fix was a motherboard loose screw.