The clicking sound is coming from the drive gear box located under the back seat. Remove the seat and try to open the top via the dash controls (this requires 2 people) when you hear the clicking sound place your hand on top of the gear box and push down slightly. The clicking should stop and the top will continue its opening sequence. Keep your hand on the box until it is done. The problem is that the plastic retaining piece holding the top's drive gear located in the gear box cover is worn or broken. I found several recommendations to fix the problem. Remove the cover and lift out the gear, inspect it, no damage proceed. Damaged see BMW dealer. You will need to find a washer the size of the inner grove of the gear and at least 1/16th of an inch higher than the edge of the gear. Place washer in inner dimension and secure cover back in place. Top should operate now. If the gear box continues to make noise additional pressure on the cover can be applied using a nut and zip ties. If additional problems are still present you may need to see a dealer