Your cat may be making a low humming noise for the following reasons:

Maybe the cat is snoring. When a cat feels very comfortable or comfortable, a low-level buzzing sound will occur, which is a sign that the cat is happy.

Maybe the cat is intimidating the other person. The sound of purring varies slightly from cat to cat, but generally it is a buzzing sound.

It may be that the cat catches a cold and has low resistance to the digestive tract, causing gastroenteritis. If the cat has not had vaccinations and further examinations, it is also necessary to consider whether there are mycoplasma, chlamydia, viral infections, and digestive tract parasite infections.