"We have an idea for something which is not exactly a bus but would solve the density problem for inner city situations," he said. "Autonomous vehicles are key... I don't want to talk too much about it. I have to be careful what I say."

Tesla has been at the forefront of self-driving technology, introducing the Model X last year with the semi-autonomous Autopilot feature. Last month, Musk unveiled the still-unfinished Model 3. With a 15-inch touchscreen tablet replacing the traditional instrument cluster inside the car, the theory is that the Model 3 will be Tesla's first stab at a fully autonomous car. Whether or not the Model 3 will serve as the basis for a fleet of self-driving buses remains to be seen.

"I very much agree with solving the high-density urban transport problem," Musk said in Norway. "There's a new type of car or vehicle that would be great for that and that'll actually take people to their final destination and not just the bus stop."

Traffic congestion is top on Musk's list of global problems to solve. He said he came up with the idea for the Hyperloop, the so-called "fifth mode of transportation," while stuck in a traffic jam in Los Angeles. And dropping hints about top secret projects in public is typical Musk too. He did the same thing in January at SpaceX's Hyperloop pod design competition at Texas A&M University, when he said he was exploring plans for the creation of an electric jet that can take off and land vertically.

Tesla Autopilot: Letting your Tesla drive itself is a bad idea