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Most likely it is a coincidence that it happened after the car wash but this can be caused by a faulty rear shock. I have had the problem once before and it was just the valving inside the shock. I wouldn't think the car wash did any damage to the rear suspension but I would let the dealership look at it and repair it under warranty as long as there is no physical damage. Please let me know if you have any questions before you decide to rate me, thanks!