According to Wikipedia, there are no isolates in China (with the exception of Korean).
However, there are several languages having disputable or unclear taxonomy within the Tibeto-Burman language philum.
First, there is (or, rather, had been spoken) Zhanzhung, an ancient sacred language of Tibetan Bön tradition which had existed in Tibet long before the Buddhist period. There is also an old Kuznetsov's theory (1988) of Tibet having some links with Sumerian culture, and since Sumerian is considered by most linguists to be an isolate, the idea of possible discoveries/researches suggests itself.
Second, there are Nungish languages spoken in Yunnan province and forming 'a poorly described family of uncertain affiliation within the Tibeto-Burman languages'. Given the vast propensity of any nations to be sinoficated within the Han borders (consider e.g. the history and background of Hui muzlims an Jews in China), it is unclear whether these languages are related to the Sino-Tibetan stock or just represent a local language union.