Cats can howl for a number of different reasons. Here are some of them:

Need for attention:

Cats may howl when they need attention or when they want to play. If they feel lonely, they can use their voice to get your attention.
food or water:

Cats can howl when they are hungry or thirsty. If their bowl is empty or if they are not satisfied with the food, they may ask you to provide them more.
Pain or discomfort:

If your cat howls more than usual, it could be a sign that he is in pain or sick. If you have such a concern, contact a veterinarian for an examination.
Anxiety states:

Cats can experience anxiety, especially following changes in the environment (such as moving house, arrival of guests or new animals). Howling can be a response to stress or discomfort.

Culture or varieties:

There are some cat breeds such as Siamese, known for their loud and frequent howls. These are cats that are characterized by a stronger vocal connection with their owners.
sexual behavior:

Cats that are not neutered or spayed may howl during courtship, especially during courtship seasons. In this case, if you are not going to castrate him, it is recommended to consider spaying or neutering.
aging:

Older cats may develop neurological problems or dementia, which can cause them to howl for no apparent reason. In this case, a veterinarian should be consulted.
The need to go outside:

If your cat does not go outside regularly, he may howl to ask to go out or explore the environment.

what to do
Listen: try to understand the reason for the whining and give him what he needs.
Attention: If he needs attention, take time to play with him or pet him.
Medical examination: If you fear that the howling is due to pain or illness, do not hesitate to contact the veterinarian.