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When you get the "thud" you would need to confirm if starter is binding or too weak or another problem.
A "thud" can be heard when power is too weak or bad connections such as starter ground or cables, when starter is binding to the point it cannot make any correct contact to turn the flywheel and or weak starter, etc.
When you get the "thud" see if it is coming from the starter area and if so check the above items I listed. In some cases some wish to just try another starter, I rather diagnose things first to pinpoint the problem but understand if that is the route you wish to try.
Also check battery is fully charged and passes a battery load test (battery load tester), check if when getting the thud if voltage is lower than 12.6vdc (not during cranking), you can also preform a starter draw test where you check the voltage during cranking to see if excessive amperage draw is a problem.
Yes I do see where you say battery is good, please understand on my end I confirm everything such as a battery load test, with the exception if battery was just replaced and both have the same problem, even then it should still be checked to confirm.
Another thing to check is that engine
actually turns by hand when starter will not, thus eliminating other engine problems.
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