If the rear brake disks are of the same type as the 97 Audi A6 I would
guess its the abs ring on the rear wheel hubs that are corroded. That is
what happened to our 97 A6 when I had this problem a year ago.
The dealer usually change the abs ring (sits on the bearing hub) when they
change the rear disks, as they are known to go bad. I however got the
disks changed by an independent workshop, which failed to do this, so two
years later I had to change the whole lot again.
On the 98 and later A6 the rear disk and hub are made in one piece, so the
abs ring get changed every time you change the disks.
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