a bottle rivnut!
Similar here.
Not a rivnut as the thread is direct in to a large alloy casting but still bottle cage related.
Before I located it I changed.
Headset bearings and wedge.
Full headset.
Frame bearings.
Frame bearings re fitted twice.
BB.
Crank.
Wheel bearings.
Fork.
Shock.
Removed, cleaned and re fitted to correct torque.
Pedals.
Gear hanger.
Seatpost, saddle rails and seatpost clamp.
Rear mech.
Frame geometry mutators.
Cable clamps.
Basically every component on or bolted to the frame.
I only removed the bottle cage as it was the one thing that hadn’t been checked. There’s no way a bottle cage can be responsible for a death creak like that though right…
Straight away I could see fret marks on the cage plate and frame mount.
One small fibre washer that was laying around on the work bench fixed it.
A fair chunk of time and money to sort that one.