Possibly, the starter solenoid is failing and engaging the starter motor before the starter pinion meshes with the ring gear. At this point it is probably best to replace the starter.
To prevent recurrence, before removing the existing starter, make sure
all of your starter wiring is in good shape and connections are tight,
including the ground cable connection to the block.
While your at it, verify that all of your engine compartment grounds are present, clean and tight. If insufficient voltage was reaching the starter, the solenoid internal contacts that energize the starter motor might have burned and distorted, causing premature starter cranking.
FYI - Randomly adding a shim for 'grinding' is the wrong approach. Proper starter spacing is based on
measuring starter pinion to ring gear lash and is generally performed to resolve singing or starter whine while (excessive lash) or immediately following (insufficient lash) cranking. For symptoms to begin right after replacing the intake is probably coincidental, unless you have poor cable connections to the battery or starter, or a poor ground connection to the block as a result.
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