A cat's low vibrating sound may be due to the following four reasons:

1. Comfort and pleasure: It may be that the cat purrs because it feels relaxed and comfortable.

2. Warning: The cat may be warning other animals not to let them get close to it.

3. Caused by illness: The cat may be suffering from respiratory or oral diseases, causing this sound.

4. In estrus: When a cat is in estrus, it may also make a low vibrating sound. At this time, the owner should be careful not to let the cat go out, otherwise the cat may run away.