The unnamed American citizen assigned to the consulate in Guangzhou had reported a variety of "physical symptoms" dating from late 2017 to April this year, the USA embassy in Beijing said in an email.
The diplomats said when they left rooms in the embassy, the symptoms and sounds immediately stopped.
The American Foreign Service Association said then that government employees had been diagnosed with "mild traumatic brain injury and permanent hearing loss, with such additional symptoms as loss of balance, severe headaches, cognitive disruption, and brain swelling".
"The US government is taking these reports seriously and has informed its official staff in China of this event", it said.
Embassy spokeswoman Jinnie Lee told USA media the employee suffered a mild traumatic brain injury while working at the United States consulate in the city of Guangzhou.
Last October, the State Department ordered non-essential embassy personnel and the families of all staff to leave Havana, arguing the US could not protect them from unexplained illnesses that have harmed at least 24 Americans.
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The employee experienced the symptoms from late 2017 through April 2018 while on assignment in the southern Chinese city of Guangzhou, where a USA consulate is located, according to Jinnie Lee, a spokeswoman at the US embassy in Beijing. The person was sent to the United States and diagnosed with MTBI on May 18.
The spokeswoman, Jinnie Lee, said the department is taking the incident "very seriously".
The embassy advised Americans experiencing "unusual acute auditory or sensory phenomena accompanied by unusual sounds or piercing noises" not to attempt to locate the source of the sound but to move away from the location, and seek medical help. Officials believe some type of sonic weapon may have been used.
Was this a sonic attack?
More than 20 members of staff in Havana were harmed in the "health attacks", according to the state department.
The US government has held Cuba responsible, arguing that Raul Castro's authoritarian state must have either carried out the assaults or at least known who was behind them.
Unlike their American counterparts, however, no Canadian envoy reported hearing any suspicious sound prior to falling ill.