I agree that it sounds like a bendix issue. A bendix works with the initial turning of the starter armature, extending towards the flywheel with the centrifugal force of the spin. The GM starter bendix were well known for the grinding sound of the bendix gears spinning too fast prior to engaging with the ring gear.
Easy fix: Take the starter out and place in vise. Use a chisel and hammer to pean the outer edge (a couple of spots) of the bendix collar to reduce the slippage when spinning. I have seen this prolong the life 2-3 years. This will at least help diagnose the problem.
Better fix: new bendix, not complete rebuild unless the starter is just tired and not spinning fast enough.
