The US government has issued an alert to Americans in China after a federal employee sustained brain injuries that could be linked to a suspected 'sonic attack'.

The unidentified American worker was stationed at the US consulate in Guangzhou when they reported experiencing "subtle and vague, but abnormal, sensations of sound and pressure".

These mysterious symptoms were felt from late 2017 through to last month, at which point the employee was brought back to the US for a medical evaluation that revealed the worker had sustained something resembling a mild traumatic brain injury (MTBI, or concussion) in the episode.

While hard facts surrounding this strange incident are scarce, there are similarities to an alleged 'sonic attack' that befell over 20 Americans working as diplomats in Havana, Cuba, in 2016; so the State Department isn't taking any chances.

"The US government is taking these reports seriously and has informed its official staff in China of this event," a health alert issued by the US Embassy in China explains.

"We do not currently know what caused the reported symptoms and we are not aware of any similar situations in China, either inside or outside of the diplomatic community."

While there's no evidence at present of other such episodes in China, authorities are advising any Americans living there who develop symptoms or medical problems "during or after a stay in China" to consult a medical professional.

"While in China, if you experience any unusual acute auditory or sensory phenomena accompanied by unusual sounds or piercing noises, do not attempt to locate their source," the alert reads.

"Instead, move to a location where the sounds are not present."

Little else is presently known about the kinds of noises the government employee may have been exposed to, but US Secretary of State Mike Pompeo told a congressional hearing on Wednesday that the markers of the latest episode are "very similar and entirely consistent with the medical indications that have taken place to Americans working in Cuba".

"We have medical teams that are moving to be on the ground there [in China]," Pompeo said, according to an NPR report.

"We are working to figure out what took place both in Havana, and now in China, as well."

The kind of mysterious noises to avoid – as heard by the American workers in Cuba, and now potentially China – sound like this, although experts are divided on whether this is actually a kind of sonic weapon, or something else.

Cuban officials have consistently denied any knowledge of a sonic attack being perpetrated against the Americans affected – who experienced hearing loss, vertigo, sleep impairment, and brain swelling.

A subsequent study of the individuals confirmed they experienced concussion symptoms "without an associated history of head trauma" – ie. without being hit on the head – but wasn't conclusive about how such symptoms could have been caused.

In that study, participants variously described the noises they heard as being like "buzzing", "grinding metal", "piercing squeals", and "humming" – and said they were related to physical sensations of air pressure, like "air 'baffling' inside a moving car with the windows partially rolled down".

Whoever you are, wherever you are, if you're hearing things like that, it's probably a good idea to move to somewhere where you can't hear those sounds.

Because whatever unknown and invisible menace is at work here, it's something neither scientists nor governments can fully understand yet – and the terrible part, sorry to say, is it looks like it could be spreading.