Just read this tread and I think it sounds like there's something slightly misaligned in the high/low lock.
In gears 1-7 the 3rd planetary gear spins to give you the low range and in 8-14 it's locked for the high range.
This misalignment doesn't cause any problems in gear 1-7.
But every time you shift in gears 8-14 this misalignment rears it's ugly head.
Something shifts a fraction in the lock when you change a high gear, it's not enough to fully work the lock.
So it still works fine under both light and heavy loads.
But enough to hear it correctly realigning itself back into place under heavy load.
Once it's aligned, it doesn't shift again until you shift gears again.
Hence no matter what you do there's no further sound until you do shift gears in the high range.

I maybe wrong but something like that is the only thing I can think of which fits the given symptoms.
 

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Tristan Ridley who is a long distance back packer has moved back to chain from belt.  He has a YT video explaining why….

Available here...
https://www.youtube.com/watch?v=nx5nN3kyx5k

Thanks. Both mucky conditions and challenging terrain. I would be wanting a parachute in case I fell off some of those tracks!

Five winters ago I used a belt drive bike on my local roads and the bike started making lots of creaking noises. I eventually traced the problem to be the belt getting dirty. It needed a thorough scrub which was more maintenance effort than putting a bit of lube on a chain. The attached photo is of the bike I've been using on the same roads for the most recent winter and shows the typical accumulated muck over nearly 1k miles. Once the winter is over I'll fit a new chain which is less effort than cleaning the dirty one and costs less than £15.

...
Re carrying/storing a spare belt - Gates say.....https://youtu.be/VgM-qeHGK_4?si=juJL4eavETswXT8s
...

I was surprised to see that Gates recommends that.  I do not recall where, but somewhere several years ago I saw that it was clearly stated that if you did that, it would damage the belt.  I might have seen that on the Thorn website somewhere.

But if Gates says it is ok, then it clearly must be ok.

There used to be a discount bike parts company in USA (Nashbar) that would pack their wire bead tires that way when you bought a tire from them, shipping boxes were much smaller that way.  Before I put a tire on a rim, I would have to unkink the wire bead in a few spots.

A decade ago on my Iceland trip, I met this couple there, see photo.  They were a mixed marriage, one had belt and the other chain drive on their Rohloff bikes.  They said they carried a spare belt because nobody in all of Iceland sold them.

Apologies just saw this. I imagine you have a frame now. I have almost finished building this up into a finished bike...

Tristan Ridley who is a long distance back packer has moved back to chain from belt.  He has a YT video explaining why….

Available here...
https://www.youtube.com/watch?v=nx5nN3kyx5k

Best, Dan.