Good news. The last time removed the road wheel, I mentioned that everything looked nice and clean. I was referring to the contact areas between the wheel and hub.
The AP Racing calipers on the GTS are very difficult to clean properly because of all the nooks and crevices that are part of the caliper design and the lack of clearance due to the wheel spokes. I can only give the calipers (and the wheel interiors) a good clean when I remove the wheels for reasons such as replacing the brake fluid. So I took the opportunity to give the caliper (and wheel) a thorough clean with Meguiars car wash and a good spray rinse with the hose. Also sprayed the contact areas between the hub and brake rotor with WD40 before replacing the wheel.
Bottom line is that the noise now seems to have disappeared, not quite sure why. Might have been a small stone as suggested by dassaur or something else. The lug nuts have always been torqued to spec and in the correct sequence using a torque wrench.