Strange "clack" on E key

A million-dollar question. It could be anything. Call the technician from the shop that sold it to you. They will surely resolve it. Hypotheses: The escapement lever is hitting something adjacent. A pivot of the lever or the part is leaking/slipping. Noise from a damper. Hammer head coming loose. Sluggishness of the part and/or the hammer (screws). Something that has fallen under the key or under the action (pencils, etc.). General obstructions to the movement of the action and the keyboard

Procedure: lift the action with one finger at the level of the rail and check if the noise comes from the key. If the key is free, hold the key and move the rail-hammer. Then unhook the spring and move only the hammer. Then move only the damper