Nope, I did not find the solution. It seems to be common, but some units are noisier than others. For me, the noise also varies from time to time. I think it depends on the force of the filament’s backlash after the buffer is refilled. Maybe the spring pressure varies a little, and the filaments have different stiffnesses (PLA is stiffer than PETG, etc.). I installed a much longer third-party PTFE tube between the buffer and the printer itself. This part of the tube moves during the print – the assist motor feeds the filament, and when it stops, the extra pressure creates backlash, the motor skips back, and it clicks. My original tube was short and rigid, causing extra pressure on the filament path. This longer tube is more springy/flexible, which seems to reduce some noise.