If a kitten is making a buzzing, motor-like sound, it could be due to one of the following reasons:

The kitten felt comfortable and purred.
Kittens growl when they are agitated or feel threatened.
The kitten has a respiratory illness and is making unusual noises.
If a kitten exhibits symptoms such as decreased appetite, runny nose, or sneezing, it is recommended to seek veterinary care promptly.