As you are exploring the nine realms in God of war Ragnarok, you will often notice what appear to be gongs. They are a faded green colour, like rusted copper. When you get up close to them, however, you will find that there is a little padlock next to them, preventing you from being able to do anything for now.
It will take a little while before you gain the ability to interact with these. Without giving too many spoilers, it will be after you have obtained sigil arrows. It comes as part of the main story so once you obtain the ability to interact with them, you will most definitely know that you can do it.
For the most part, these gongs will lead to some chests that contain handy crafting items such as ingots and some hacksilver. They are definitely worth seeking out if you are planning to reexplore past areas of the game after completing the main story. You have likely spotted a lot of those steam vents that appear to lead to secret areas and even the glowing cracks in some areas of stone. This is a good incentive to go back to early areas to see what you may have missed. Can be a useful thing to do before you go ahead with the final mission of the game as they will give you everything you need to get stronger in combat, every little helps!