You're paraphrasing there, There was no way you would have knowingly sold me a dodgy printer, having met you and seeing your projects / reading you blog I don't think that would have been the case here (not have I ever said that it was).
Yes, the noise is gone for now and has been printing ever since. (Fantastically may I add).
To help others let me say what I did:
I think the stepper may have slightly skewed and that then skewed the belt, not a lot but enough to create uneven tension. A search reveals lots of people with issues in that area.
There was also a slight buildup of rubber in the grooves which I cleaned out with some ISO and so that may have also contributed to the belts not sitting right.
I also cleaned and oiled the stepper itself, which had an immediate change of feel.
I think you're right Will, the process of packaging may have jolted it, it would also be pretty easy to catch the belt with fingers when removing from the box so it may have even been me that made it worse!
Either way I think we may have solved the intermittent problem by solving the exasperated problem.
When I'm over the honeymoon period of printing lots of things with ridiculous quality I'll probably take down the other side and give that a service too.
Just to conclude this nicely, here's what it sounds like now: https://goo.gl/photos/Y4aihrqQczxKqg739