My cat has been making sad and mournful cries at varying times of the day. He is a neutered 15 year old.?

He is in good health, as he has been to the vet twice since September, with blood tests (we were concerned about Feline Chronic Renal Failure, but all levels were good). He's loving and doesn't seem to be acting any differently other than the caterwauling. Sometimes he will walk away after being fed, petted or otherwise interacted with and start these sounds. He could be less than 10 feet away from us when this happens, or on the other side of the house. Sometimes he'll wake us up the middle of the night. If we call out to him, (ie "Baby, we're here") he'll stop. He has access to us, food, water, litter box and toys. My vet is unconcerned, but the noise is rather irritating. Any help or suggestions?