That's awesome! My Sweety makes that sound too, which he has been doing because he's mimicking me! I have a little pattern of a sound I've repeated since I got him, it's like "kiss kiss kiss wolf whistle". A few weeks ago I noticed he was definitely attempting the kiss kiss sound, and a bit after that I heard him attempting the wolf whistle as well. When we are just sitting around sometimes and he is not otherwise engaged, I'll hear him start practicing either or both sounds, and I will start softly doing the sounds myself. And he clearly does a back and forth with me, it's so adorable. It is just that he is still practicing at it so it is difficult for others to understand what that sound he's making is. So when someone is over and he does it and I'm like "isn't that cute how he makes kissy sounds" and they are like "umm....not sure, I can barely hear it". 
