https://www.vice.com/en_us/article/bvjb75/scientists-have-solved-the-mystery-of-a-strange-signal-from-earths-core-study-reports

Scientists in China say they've solved a major mystery deep inside the Earth by confirming that the inner core wobbles with a predictable rhythm that repeats every 8.5 years. 

The research team says that the finding confirms previous measurements made by the team in 2018 and challenges earlier assumptions about the relationship between Earth’s layers and how their movement affects things like our changing day lengths. The study was published in December in Nature Communications.