Sounds like the starter is sticking rocking the car in gear can help to get it started but yeah imo buy a new one ready for when it gives up the ghost!
Whip the starter out and have a look. It may be something as cheap and easy as new motor brushes or just a dam good clean up. They can get full of clutch dust over time.
this. Mine did the same thing after I didnt start it for three weeks or so. Hasn't done it since. The noise is the starter moving but not moving out to contact the flywheel.