Lens Autofocus Problem?

I bought a 2nd hand sigma 17-50 lens from someone for my nikon d5300

However when I tried the lens on the camera there was just one problem.

When the lens is set to auto-focus, the display of the camera would focus rapidly between af-s and m. And also when i half press the shutter, the beep noise from focusing doesn't happen.

The manual mode of the camera works fine.

The internal focus is also not broken as the lens managed to auto focus for 5 seconds until it went back to the problem.

Also my other lens works with the camera just fine only this lens.

And I don't think it is the contact problem of the lens otherwise the camera wouldn't be able to control its aperture. And the camera doesn't say lens not found

The lens also has a built in auto focus motor

The lens is faulty - you got taken. It should never switch between the two on its own.

Return the lens.

It is faulty and the cost of repair may make the cost of the lens, what you paid for it and the repair to cost more than a new lens.

My guess is that you purchased this lens from an eBay account. In the future only purchase used camera gear from reputable sites like KEH.com, MPB.com or the used departments of B&H Photo and Adorama

Lens problem. Maybe internal or the motor itself. Return the lens and ask the seller for a refund.