The Old Rattle-Drum is one of Black Myth: Wukong's more mysterious key items. You'll first acquire it during chapter two after you defeat the Tiger's Acolyte boss on a bridge in the Yellow Wind Formation area, having travelled through the ruined Windrest Hamlet beyond Crouching Tiger Temple. It's a strange item for a swordsman to drop, and it doesn't offer any clue as to its purpose.
It does serve one, though, even if it took me way too long to puzzle out what it was. You need to play the Old Rattle-Drum in certain locations in chapter two's Yellow Wind Ridge area, and if you find them all, it'll unlock a secret boss and some special items. So, here's what to do with the Old Rattle-Drum.
There are three locations where you need to play the Old Rattle-Drum in order to unlock its secret. When you get close to each, the screen will go grey, and you'll hear a young boy calling out to you. Keep approaching and you'll see an action prompt that lets you play the Rattle-Drum. This will attract undead in the area that you have to defeat, but as you do, the young boy will tell you his story.
The locations in order are:
After playing the Old Rattle-Drum in this last location, a child will appear and hop down the well, bidding you to follow him. Interact with the well to drop down and you'll find the Mad Tiger boss in a cave below the village. He isn't that different to the Tiger Vanguard boss so shouldn't give you too much trouble.
Once defeated, absorb the Mad Tiger Spirit and open the chest at the back of the arena to get the Plaguebane Gourd, which increases attack whenever you drink from it. With that, you've discovered the drum's secret.