I got a new bike a few months back. Its a cheap decathlon Rockrider mountain bike so I am not expecting anything too special. However I have noticed that going down steep descents, even slowly, the back disc brake makes a sort of knocking sound, tap tap tap all the while I hold it down. Even if I am going slowly it happens if its steep enough. Going fast and using it on a flat surface it doesn't seem to happen. It's my first bike with disc brakes. Any ideas what's going on?
Possibly calliper alignment. Manufacturing variation in hub or badly faced rotor mounts can move the rotor plane in or out a crucial tiny amount which combined with a odd calliper shape can result in some of the arms of the rotor bonking the calliper under the right conditions. Hence the thunk, thunk sound. Not dangerous but annoying.
Moving the callipers a touch to one side (e.g use a paper width as a gauge) might fix the issue without significantly affecting the centering.