what if I hear whirring from starter?

Question

when I touch solenoid with screwdriver I hear starter turning with whirring sometimes ginding noise, took starter off works fine. dont know now. please help

Answer 1

Always give year, make, engine size and model # when asking questions. We need all of that info to give an informed answer.
From your description - bad starter clutch.

Answer 2

Solenoids

To achieve this a solenoid is mounted above the starter. Some purists call this bit an actuator, but no one in the motor trade does!

When the starter is switched the solenoid pulls a plunger into itself. The plunger is connected to an engaging lever that in turn is connected to the starter gear (known in the trade as ‘the drive’). So switching the starter pulls a plunger in which pushes the drive out into the ring gear.

At the top of the solenoid are a set of 3 contacts. One comes from the battery, one goes to the starter motor itself and between them is a moving contact. The plunger (which I talked about in the previous paragraph) comes back and pushes the moving contact onto the others, thereby connecting the battery to the starter motor.

So the motor now turns, however as the drive is now engaged in the ring gear before the motor spins, it doesn’t wear out the gears.

On pre-engaged starters the drive has an internal one-way clutch. This slips in one direction and locks up in the other. This is again designed to prevent the starter running with the engine as it helps to stop over-speeding, because when the ring gear is going faster the drive it slips.