Hello hoping somone has seen this issue before. I have a 2000 blazer 4.3 v6. It has had this starting issue for a long time. The starter will actually engage and disengage while cranking the engine. Sounds pretty horrible almost like it was throwing metal. I have replaced the starter. Flexplate. Batery and cables. Replaced starter relay. I have no engine lights to indicate any electrical fault and have loaded wires associated with the circuit. This problem also does not happen everytime i try and start it. Teeth on the flex plate are still good not broken or ground off. Starter is only about a month old still new

 

Since two starters do the same thing and the gear and plate teeth look OK I would rig up a wire to the post on the starter with the purple wire and get a voltmeter or test light on that wire and see if it loses power intermittently during cranking. If so then we work backwards towards the battery to find the cause.

If your a parts cannon guy then I would go with the ignition or trans switch but I prefer proper diagnosis myself. I'll help you whichever way you decide.

George