Subaru XV 2014: Engine making unknown noise

Of late, whilst driving my 2014 Subaru XV, it sounds as though it is stuck in a gear. My rev counts are low, but there's a noise as if it needs to change gear (it's automatic). Almost as though the engine is running too high. Have taken it to my licensed Subaru mechanic and had it tested, everything came back perfect. He could hear the noise, but couldn't identify it exactly. He advised that one could spend a lot of money trying to find the problem and still not find it. Instead wait until the noise is really noticeable and go from there. It has 128,000 km, no other problems to date. Any help would be appreciated.