Unexplained noise
 

Hi, I’ve had my 2014 Mini Cooper Roadster for 5 years now, some months ago it developed an intermittent noise which was just like driving over loose chippings - as if they were being thrown up under the car. It happens at speeds between 50 & 70 ish. Has anyone else experienced this? The BMW/Mini garage today couldn’t identify a problem and could only suggest a 2 day stay while they took it apart! It doesn’t affect performance and no warning lights come on. I’m hoping someone here has the answer.