Wall mounted electric fireplaces that use a conventional fan heater can be quite noisy during operation. However, most electric fireplaces can be used with just the flame effects only, which can be very quiet, and some models will use an infrared heater instead of fan forced, which are silent.
Our own wall mounted electric fireplace has a fan heater and so makes some level of noise during operation.
The noise is generated by the blower rotating inside the unit.
We have other electric fireplaces that also use a conventional fan heaters with blowers and so we knew what noise to expect with this particular electric fireplace.