That knock is completely normal for a Roller brake. It's part of the braking mechanism, rather than the torque-limiter.
Look in the notes of the Shimano technical docs.
If you apply the front Inter-M brake strongly while the bicycle is
stopped and then shake the wheel, you will notice that there is a small
amount of play in the brakes. This is normal, and will not cause any
problems at all while riding.
All the best,
Pause, take a breath and ask yourself: "What would Rastamouse