Stumped by Loud clacking on 2nd gear of stock 6-speed Brompton :(

Hi guys, I’ve been trying to sort this out and have hit brick walls so far. Maybe someone here has encountered the same issue.

Background: 1 month old brompton s-bar, 6 speed with a rack and mud guards. Manufactured in 2023. Developed this noise 2 days ago after a short hard cycle over some bumpy back alley roads.

Issue:

* This loud clacking comes off and on, but is more pronounced over slower speeds and only when pedalling under load.
* There is no clacking when bike is raised and pedals are spun manually
* Happens on both “+” and “-” but ONLY on 2nd gear. Never the 1st or 3rd

Tried:

* I’ve adjusted the chain pushers; no difference
* I’ve adjusted the gear indicator chain; no difference
* I’ve checked the chain itself and it looks aligned
* I’ve brought this over to an official Brompton shop and the mech says he has no clue. He tightened the rear wheel assembly but the noise, while lessened, is still there. He also agrees its not from the internal hub.
* I’ve checked the pedal nuts and chainring nuts and they’re nice and tight.
* I’ve removed all unnecessary accessories (bags, pouches, toolkit) to remove rattle — no difference

Bike is clean, chains are lubed, and I am stumped. Has anyone come across a similar case?