Pre-engaged starter?
Turning key to start position firstly throws the solenoid across, engaging starter pinion with the toothed ring on the flywheel. At the same time the solenoid is throwing in the starter pinion, it also provides a connection to the starter motor which will then turn.
So, it could be a failing solenoid or starter. It could also be insufficient current capability to turn the starter when the pinion is engaged...
The only time I've had that failure (on a V8 plumbed into a SIII Land Rover, it turned out to be the starter - but I'd still check everything already suggested.
Cheers,
