Base resistance missing in BC817-40 datasheet
The real issue here is that you are worrying about noise in a BC817 (aka BC337 in SOT23). They are called general purpose amplifiers but they are really meant as switching transistors. They even give the thermal transient curve for that.
Probably it's not there because either a) it's not something to write home about or b) it's not a process controlled spec so it could be anything at the end of the day.