I hope everyone is getting some good use out of there vans in the sunshine

I was coming of the motorway in mine after about an hours drive and as I slowed down fairly rapidly as it was into the services the buzzer went off. I stopped and when I started again everything was fine. I actually hit a massive pothole and though the bash might have set it off.
Since then every now and then it randomly comes on when i come off the motorway. Not always. Like 1 in 10 times ...
Its a 1.9 water cooled DF
Anyone have any ideas .. ?