Modern mountain bike transmissions are 10-speed. Part of the fun is that they require a narrower chain, like the SRAM 10 series. These are, on paper, some of the best chains ever made. After all SRAM’s chains have a rep for being tough, and with the Powerlink technology, about the simplest to use.
However, when these chains are new they seem to run noisily, even on perfectly tuned brand new SRAM running gear. It doesn’t affect the shifting or drive performance, but it’s an annoying backing track for the first few hundred miles. The good news is the noise abates with use, almost like it has to wear in a bit and then it’s as good as all the rest.