I showed up at our shop one night when we put together the new car.. and it had a noisy starter. The guys shimmed it with a little and a lot made no differnece and even made it worse.. I sat there and said I knew a quick fix and that I could make better...
I put a 3/8" washer on outside bolt between the block and the starter.. Tightened up the inside bolt 1st and then the outside bolt...spun the motor over with no noise.
I do not know why this happened to blocks and starters but I have seen it on race cars and my own personal 1 ton.
If the starter does respond to the fix the problem is that the starter is too far awy from the flex plate. I take the starter off and file the starter down .you really cannot file too much because you can always shim it back
good luck