Brompton M3R - crackles at the hub

Hello,

A new B owner with 3-speed... This is my first hub gear bike. When I ride on 2nd and 3rd gears, the ride is seamless, but when I change to lowest 1st gear, especially before a slope, there's a crackle and clatter sound/feeling from the hub. Like when you pedal, there's short "krak!" and...