It's normal for an auto. Learn to shift the tcase quickly. The trans output shaft will turn any time the load is removed from the trans output, causing grinding. Trans in PARK puts a load on the output shaft. Having the tcase in gear and tires on the ground is a load. PARK is the only position the output shaft won't turn with the engine running. The grinding when shifting to PARK is the parking pawl in the C4 grinding on the park gear. It happens after the output shaft has been allowed to come up to speed because the load has been removed. Don't do that, it can break. Sometimes your only safe option is to shut the engine off to stop the output shaft. Turn the engine off, put the tcase in gear.