Agree it does sound a bit strange - I did wonder if they just replaced the hub and wheel from another bad boy but it is unlikely a chain that large would just conjer up a random story!

The sound was more like that of a popping noise - a bit like when you squeeze a bubble wrap and pop the individual bubbles - every time the wheel rotated over that same spot it made the nosie and it might also explain why it only happened when full weight was on the tyre. The mechanic put all of his weight on saddle and rolled the tyre over the offending area and that was when the noise occured.

I will speak to the mechanic on Monday to ask him exactly what it was that caused it so that if I ever change a tyre again I know what to look out for.