I had the similar problem which turned out to be a bad lock actuator. Its a good thing its under CPO warranty. It also could be a faulty lock mechanism. If it centrally locks with the other doors, it could be just the mechanism. Do a search on door actuator in the E53 forum

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03 E53 (retired)
07 E70 3.0
07 335i....300hp
05 330ci ZHP