A high-frequency sound from the AC adapter like HP ED494AA is normal. The sound may often be too high-pitched or too quiet to hear, though it may increase in volume after long periods of connection. And If the output of an adapter exceeds 120 Watt,there would be a built-in fan for the adapter to cool down the main box. In some cases,the fan maybe annoying but it's really useful for people to get rid of fire hazard.
Some adapters come equipped with an internal sound, often a consistent beeping, to warn the user of incorrect operation. Adapters acting in this way may not supply power correctly to the laptop, and should be replaced.
While some noise is normal, a sudden change in the consistency or type of noise may be indicative of a problem. A snapping sound, for instance, can accompany an electrical problem. HP ed494aa Adapter displaying abnormal noises should be repaired or replaced, as damaged adapters pose a risk of electrical shock.