Does your dog talk or sing? It may be something your dog just likes to do. Or perhaps some sound annoys him or triggers it, such as the piano or the theme song to Law & Order. Or perhaps it could be something you taught him, whether purposely or inadvertently.
I inadvertently taught Sephi how to ‘talk’. She made cute sounds when he was a pup and I thought it was so cute that I encouraged it. Be careful if you encourage your dog to make noises. Your dog may be inclined to do it more often in order to get your attention. Sephi, for example, now makes noises to remind me that it is dinner time. Sometimes it is annoying because it is too early in the day for dinner. Or sometimes she gets carried away and barks, which is not so cute.
Tell us if your dog talks or sings. Does he do it often or only on command?