96 chevy s-10 blazer

96 chevy s-10 blazer has issues when starting after ignition is engaged there is a thump sound that prevents a proper start. This is a new issue and the Blazer will start after a few tries. I changed the starter without a change in results and started without the serpintine belt without a change in results???

Some vehicle starters require spacers to adjust the distance the Bendix gear has to travel to safely engage the flywheel.