Why does the Xiaomi 15 Ultra make a buzzing sound when vibrating?

Thank you for your support of Xiaomi products. The Xiaomi 15 Ultra has undergone strict professional inspection before leaving the factory, so you can use it with confidence. 
The phone is equipped with a newly designed motor that offers several enhancements. This upgraded motor ensures a more uniform and consistent vibration experience, providing smoother feedback when interacting with the device. Additionally, the motor has been engineered with weight reduction in mind, making the phone lighter without compromising its performance. As a result, you can enjoy an improved vibration experience that is both more reliable and satisfying, while also benefiting from the device’s enhanced overall efficiency.