Thanks for the replies both. After finding this other thread it seems clear that it’s what I’m facing too:
Zwift hub “belt” Crooked?. TLRD; the belt is misaligned/slipping and rubbing on the adjacent plastic.
On one hand it’s incredibly refreshing how little friction there is (ironically) with Zwift’s support and getting replacements organised, but also quite telling about how aware they are of the disappointing effectiveness of their quality controls.
Let’s just hope that third time’s a charm!