Capacitors are sensitive to audio frequency and can pick-up and emit said energy. Here’s an app note from KEMET about these. Link is here
Multilayer ceramic capacitors (MLCC), made with class 2 or class 3 dielectrics, typically exhibit a piezoelectric effect when placed under electric field. The piezoelectric effect results in a physical distortion with amplitude related to the intensity of the electric field. If the electric field is an AC field with frequency in the audible range (ca. 20 Hz to 20 KHz), the distortions due to the piezoelectric effect may result in sound emission. If the MLCC is soldered directly to a circuit board, the sound emission increases analogous to a speaker. Additionally, it is not atypical for several MLCC on a circuit board to combine to further amplify the sound emission to the point that it is noticeable, if not annoying (i.e., ‘singing capacitors‘). The inverse of this effect is that the MLCC may also act as a microphone when subjected to shock or vibration, thereby adding noise to electronic signals in a circuit, for example in the case of a high vibration application.
This paper discusses options for reduced sound emission and microphonics, including high capacitance per unit volume C0G MLCC, lead frame class 2 X7R MLCC, lower distortion class 2 X7R MLCC and Ta capacitors. The various options are compared and contrasted in order to determine the most suitable solution for the intended application. Acoustic data for lead frame class 2 X7R MLCC, lower distortion class 2 X7R MLCC, and standard class 2 X7R MLCCs are also presented.