They did say the original carrier (non slim) had worn on the sides, and changed for a slim carrier to improve chain line.
Now, by a process of elimination, we're probably there. It's very likely a still-skew chainline, with the effect aggravated by an extra-thick Thorn chainwheel, an extra-stiff X1 chain, and the extra width imposed by the new sprocket fixing method (even the slimmer one is wider then the original threaded one). What you're hearing is rather likely to be the corners being worn off the teeth of your gears by your stainless chain.
You need to fix your chainline by one (or more in combination) of these measures:
*spacers to bring your chainring into a straight line with the sprocket,
*refitting the bottom bracket with spacers to bring the chainring in line with the sprocket,
*or, possibly, if the chainline isn't too far off, a thinner chainring might be a temporary bodge (fit your old chainring temporarily and see if the noise goes away)
MEASURING THE CHAINLINE
The chainline is the distance from the centerline of the bike to centreline of the chain.
To measure the chainline take a measurement with the chain fitted at the front over the chain ring and at the back over the sprocket.
You might find your wife (or mother in law's) hemming measure a useful tool.
To use the bike frame and component hard points (as distinct from guesstimates about centres) as your references:
FRONT: Use vernier callipers to measure from the left side of the seat tube to the right hand side of the chain. Measure the tube diameter and the chain width separately. Subtract half the tube diameter and half the chain width. The result is the chainline at the front.
REAR: You should not have to measure at the rear because the chainline there is a published number you can look up for all the available and heritage sprockets.
RESULT: The difference between these two numbers is the distance you must move the chainring for a straight chainline.
