I think you mean "purr". That's a noise that the cat makes.
the sound made by a happy cat
Assuming you're talking about a phrase like 'the motor purred like a kitten'. It simply means the engine is so perfectly tuned, that instead of a noisy, rough engine sound, it produces a smooth, quiet 'purr' note. Think of it like comparing the engine noise from a 10-year-old Ford, against a brand-new Rolls Royce !