Our visitors from London drove their two-month-old small hatchback down the motorway for the first time. An alarming intermittent bang at speeds over 50 mph, apparently from underneath, kept their speed down. Investigation eliminated fairly obvious causes such as an unsecured spare wheel. Into the local main dealer hopefully for repair under warranty and they managed to replicate the noise on the road, but could not locate it on their ramps. They kept the car overnight. In the morning came the call, problem solved. A small ball, about the size of a ping-pong ball, had been placed on the end of a rather whippy roof aerial. At speed this was being forced down by wind pressure and banged intermittently on the roof. The trouble was not underneath at all. Our friends were relieved and able to laugh, but unfortunately £68 lighter. The ball made it easier to spot their silver car in a car park and had worked successfully on a Polo and a Clio in the past. The new aerial was whippier and nearer the roof .