Traditional heat sinks for electronics cooling have become ever more difficult to design to meet the high dissipation rate of modern high-heat-flux electronics. Active devices, especially devices operating at a high frequency, show promise toward enhancing heat transfer performance. However, active devices generate noise that may not be acceptable to personnel. The present work studies acoustic characteristics of piezoelectrically-driven synthetic jets and oscillating plate agitators operating at high frequency (around 1000 Hz) employed in an electronics cooling module for heat transfer enhancement purposes. The A-weighted noise level from such actuators is measured and found to increase with increases of driving voltage and operational frequency. The measured sound pressure level of the active devices used in our present enhanced heat transfer module can be as high as 100 dB. Through a power spectrum analysis, we find that most acoustic energy is in a narrow frequency band close to the operating frequency of the active device. To decrease the noise level, a muffler, which also allows cooling air to recirculate through the equipment cabinet, has been designed and tested. An analytical model is employed to select the geometry of the muffler for optimal performance based on acoustic characteristics of the active devices and the through-flow pressure drop. The muffler having this optimal design is fabricated and tested and found to be able to decrease the noise level generated by two actuators from 83 dB to 64 dB.