why does my cat howl before laying down?

he is 17 but in good health except for hearing loss and smaller kidneys (we go to the vet regularly and they said he should be fine and not in any pain) He seems perfectly fine, so what does it mean when he meows/ howls super loud when he is alone somewhere in the house? for example, he did it on my bed and when i come get him, he is fine, does a normal meow at me, then lays down. He just did it again on the chair in the living room. i always run to find him when i hear that (it can be heard no matter where i am in the house it's so loud) to comfort him, but i am just wondering why he does that.