The car is an October 2018 Hyundai i30 estate with 32000 miles showing. The grauching showed up in one specific part of my commute home - after braking to come off the motorway on a downhill sliproad , when pulling away onto the roundabout the brake would graunch. By the time I got home after 3 or 4 miles of town centre driving, the grauching was gone. The graunching also stopped if the brakes were applied.
The brakes are typical for this type of car - a single piston sliding caliper gripping a fairly substantial ventilated disc. When I removed the wheel & caliper, everything was free, the pads are fine & the disc contact faces are clean - no grooves, corrosion or signs of excessive wear. But I did notice a narrow polish mark on the rust on the edge of the disc & I presume that this is the cause of the grauching - the disc gets hot after braking down from 60-70 mph on the slip road, expands, and the rusty edge is just contacting the caliper bracket. By the time I get home the disc has cooled and shrunk back to give clearance. Application of a "toffee hammer" (1/4lb ball pein hammer) to the rust build-up shelled off around 2 mm of rust build up from the edge.
So far, so normal - the edges of discs rust. But what was a bit strange was that only half the disc was affected - the edge from the ventilation slot towards the suspension. The outer edge (ventilation slot towards the wheel) was surface rusty, but there was insufficient build up to knock off. Both discs showed the same pattern, though the NS disc build up was not enough to touch the caliper bracket.
Any ideas as to why this might have occurred ? The only thing I could think of was that the inboard side of the disc runs hotter, due to being shielded from the airflow by the stone guard, whereas the outer face is more open due to the slots in the alloy wheels. And maybe a hotter face corrodes more quickly ? I have not seen this on other ventilated discs on other cars, though.