When voltage is applied to an MLCC, the dielectric element expands in the electric field application direction and contracts in the vertical direction, causing the chip mounting board to vibrate and may generate acoustic noise.
By contrast, the dielectric elements used in the ECAS series do not distort even when voltage is applied, so acoustic noise caused by that does not occur.
You can also view a video showing the main causes of acoustic noise and ways to counteract them. Be sure to watch it to get an idea though images of phenomena that may be difficult to visualize, such as vibration of circuit boards on which chips are mounted.