What make tyres do you have?
Are they "feathered"?
Good point, I had considered the tracking, as it all gets quite noisy in the low frequencies below 10mph. However, all seems fine on the tyres. I swapped one over for the spare (different tread pattern and no wear at all) but made no difference at all.
Tyres are Michelins and treads are as smooth as could be expected.