At about the place where you turn off the M42 to the M40 I started thinking there was more vibration than usual. Then I thought: what's that racket? There isn't usually this much road noise. So I pulled off at junction 15 (Warwick). I stopped at some traffic lights, and when I pulled away again there was an ominous scraping sound coming from the front of the car. After driving around a bit with the window down to verify that yes, it was loud and it wasn't going away, I parked in a pub-restaurant's car park and called the RAC out for the second time in three months.
RAC man turned up in about 20 minutes, drove the car round the car park and said that with that noise it was one of two problems: a stone in the brakes, or a worn-out wheel bearing, and he sent us inside while he took the wheel off to have a look. A short while later: the offside front wheel bearing is completely shot and it's a good thing I came off the motorway when I did. They had to send out a recovery truck, which took another hour.
At least I am now at home with the car parked outside, but clearly I will have to ring the local garage in the morning. :-(