I adopted two rescue kitty Sisters from same litter when they were ~seven weeks old. We sadly put one of them to sleep several months ago at age 17 years, due to complications of chronic kidney disease. Her Sister showed signs of missing her even though they were not cuddle buddies, but had always been inside cats together. She searched and cried, seemed disoriented and didn't eat well for about a month afterward. She continued to be loving and cuddle close to me especially during evening and during night. But she started a new habit of waiting until I'm out of bed for the day, then digging under covers to make a nest in blankets, making a howling noise until she settles in. The howling part is what I don't understand. She has been recently examined by our veterinarian and had Senior cat lab tests. She is very healthy with the exception of minimally-elevated kidney function tests, which doesn't surprise me with her age and her Sister's history. She really acts a lot like a smart, sharp kitten. Can anyone help me understand the reason for her yowling while burrowing? I have a feeling it could still be related to losing her Sister as her Sister used to sleep right where she burrows. Thank you.
Some cats want an "answer" when they meow (perhaps as reassurance that they are not alone), and they will meow until they get one. In the past, her companion may have answered her. I suggest you try "answering" her. Just say "hello" or anything you like; you don't have to meow unless you want to. She probably just wants to hear a voice, animal or human.
Given her age, it might be hearing loss that causes her to yowl. She may not be able to hear her own voice. If the problem persists, you might check with the vet about this possibility, if the vet didn't already check for it.