I still haven't found the Carol, had lots of lovely French carols suggested but still not the one I'm looking for.
Helpfully don't know any of the words.
I know it is French. I think it is traditional.
I know I've watched school choirs sing it on YouTube.
It has the same haunting quality of 'Carol of the Bells'
Whispers at some points and builds to a crescendo.
My embarrassing attempt to put into words what I hear in my head...
'Nous somme, nous some dupee, enchant ...dum dum dum da dum'
The nous somme is repeated and whispered at time.
*for avoidance of doubt I can't speak French