Occasionally when my sister-in-law turns the ignition on her VW Polo the starter motor does not turn but instead the car produces a series of very loud bangs. Typically this consists of 8 - 12 bangs with increasing time between each bang e.g. Bang - half a second - Bang - half a second - Bang - half a second - Bang - one second - Bang - one second - Bang - two seconds - Bang - two seconds .. After this the car starts normally with the starter motor kicking into action. Because the problem is intermittent the VW engineers were unable to identify the problem. The starter motor has been replaced but this has not cured the problem. Has anyone seen this type of problem before. I've checked a few forums but have been unable to find any similar cases.
Many thanks