Hi,
This is something that has baffled me for years, 54 reg Touran, 1.9 tdi. Select reverse, then just as the clutch starts to bite there is a quiet but audible click somewhere in the center console area inside the car. The only way I can describe the soundit is like a butterfly valve snapping shut in a plastic pipe. You can up and down with the clutch as many times as you like but it won't do it again until you've moved forward, then into reverse and click. It's done it ever since I've had it so I can't believe it's a fault as such, suspect more of an actuator setting or adjusting something in the vent system perhaps. But I'd love to know what!!!
Any ideas?