Can you tell what’s wrong with my mtb?

Hi guys! 😊 I'm trying to find out what's wrong with my bike. There's a weird like scratching/rattling noise coming from the bottom of the bike frame when I'm using rear brake. It sounds like something is wrong with the cables maybe? I think that the noise isn't coming from the brake and the brake brakes well all the time so it does not affect the function of the brake. The noise is happening ONLY after I'm going through some rough downhill terrain (lots of rocks, sticks and lots of braking). When I leave my bike and use it another day the noise is gone and it doesn't make that sound at all when I ride on a normal road. The bike is one year old and brake pads are ok. You can hear the noise in the attached video. The noise is most noticable when the bike almost stops.

Thanks a lot in advance and wish you all good luck on the road!