Cats will hum when they are sleeping, nervous, uncomfortable, etc. Cats are very insecure animals and will hum in a low voice when they are in a strange environment. The owner can stroke and comfort the cat appropriately, or feed the cat Favorite snacks to divert its attention. Sometimes cats will make this sound when they are injured. After the owner discovers that the cat is abnormal, he should send it to the pet hospital for examination and treatment in time.