Hear a rattling sound between 4-6k rpm on your CBR 600 F4? Sound like a ring of keys jingling below the tank? Could be the cam chain tensioner.
Here’s a handful of threads on the diagnosis:
TwoWheel Forum on CCT
F4i Cam problems on TwoWheel Forum
The joys of the F4i cam tensioner on Sportbikes.ws
F4 Buzzing sound on SportBikeWorld