The dual-speaker feature phone adopts a two-in-one design of the earpiece and speaker. The speaker has a larger amplitude and a wider sound range. The top speaker and earpiece use the internal space of the phone as the speaker cavity. When the phone is placed outside, the top speaker causes the airflow inside the phone and conducts conduction. A slight vibration is caused on the back cover, and the vibration is an intuitive experience of sound improvement, which is a normal phenomenon.
If you are sensitive to this vibration, you can use a protective case to reduce the vibration, or slide the default button on the phone Settings > Additional settings > Accessibility > Channel volume balancing (or left and right channel volume balance option). This will dampen the sound of the upper speaker and slow down the vibration of the back cover, and may slightly affect the effect of Dolby Atmos.