Unusual sound from under hood

I have noticed lately that there is an unusual sound coming from under the hood. You can hear it when the car is idling, it is harder to hear when the car is driving. I am trying to get an idea of what it might be, to see if it's something I know how to tackle myself. I will add a video to this, hopefully you are able to hear some of the sound. To me it is sort of a metallic like noise, but I find it hard to explain.