Had the same problem on mine, took it in to get looked at. Was told, it is the gearbox but as it is only in reverse there is no point doing anything about it as it could last for years (five in mine) as reverse has such low mileage use. My box eventually gave up last Christmas due to bearings in main drive not reverse, got it reconned and no more noise now. Basically I was told "don't fix it if it is not broke" yet!