purr like a kitten

1. Of an engine, motor, or other piece of machinery, to run very smoothly and efficiently (often signified by a smooth, rich humming noise, not unlike that of a cat's purr). Once get this carburetor repaired, this old car is going to purr like a kitten again. I just replaced the motherboard on my PC and installed a bunch of new RAM—now it's purring like a kitten.

2. Of person, to sigh, hum, or moan with contentment or pleasure. The kids purred like kittens as they ate their ice cream cones. I know a few tricks that will have your partner purring like a kitten.