This easter egg does not give you any kind of achievement or anything else, it's just a small interesting thing in DOI that everyone should know about in my opinion.
So the easter is in the Shooting Range map, load up the game and go to close-mid distance range (with tin cans, helmet targets and etc.).
Go near four hanging targets:
And shoot them in this specific order (look at sh*tty artwork for numeration):
From left to right: 3,4,1,2
This way you will get very famous Big Ben's Chime (Video for comparison):
https://www.youtube.com/watch?v=juT1zsim6es

...for adding such small but cool easter egg and for revealing it at their "Shooting Range Reveal" stream.
Cheers