The Boos that inhabit the Last Resort Hotel in Luigi's Mansion 3 carry a hidden secret, as the HD Rumble is secretly spelling out their name in Morse code throughout the game.
The villain of Luigi's Mansion 3 is King Boo, who brought some of the Boos from the Super Mario Bros. series with him to hide in the Last Resort Hotel. There are sixteen Boos hidden in Luigi's Mansion 3 and they hide in random rooms on each floor. The best way to find them is to approach each container and feel the motion of the HD Rumble. The Boo will be hidden in the container that prompts the biggest reaction from the HD rumble.
It seems that there is more to the HD Rumble than meets the ear. A Reddit user named Jamthom8 has pointed out on the Gaming Details page that the motion and sound made by the HD Rumble when approaching a Boo for the first time actually spells out the word "Boo" in Morse code.
Luigi's Mansion 3 has several optional objectives and capturing the Boos is probably the easiest one, as it's possible to buy items from Professor E. Gadd that give away their general location. The Boos themselves are all faced in the exact same way and the reward for catching them isn't that appealing, but the fun comes from the chase and hunting them all down.
Luigi's Mansion 3 does a great job with the HD Rumble effect throughout the game. It's a shame that the Nintendo Switch Lite doesn't have access to the feature, as there doesn't appear to be any alternative hints for finding the Boos. The same is also true when using wired controllers that lack any kind of rumble feature. Super Mario Odyssey added a feature that helped players find hidden Moons that relied on the HD Rumble with a visual indicator, so it's possible that Luigi's Mansion 3 will add a similar feature in a future update.
Luigi's Mansion 3 is available now for Nintendo Switch.