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.

Where to play 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: 

• Windrest Hamlet: From the Windrest Hamlet Keeper’s Shrine in Yellow Wind Formation, head through the village as usual until you arrive at the gate where the three Rat Imperial Guard—red-robed rat enemies—jump down and attack you. Before going through the gate, head right into the courtyard with the undead gathered around a skeleton.
• Valley of Despair: The second location is easiest to access from the Valley of Despair Keeper’s Shrine in Sandgate Village. Open the big gate in the King of the Flowing Sands boss arena using Sterness of Stone and Keeness of Tiger, then head down the stairs and turn right at the bottom through the ruined house into a little clearing. 
• Sandgate Village: The final location is right next to the Earth Wolf boss arena, so you’ll want to explore the village first and defeat that so you can unlock the gate with the rat archers on top from the other side. Once open, head through from the Village Entrance Keeper’s Shrine and turn left. Follow the path around the corner to find a well with some corpses.

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.