Mine does that as well on the level surface in the garage, it will move against the lock while the engine is running, I always use my parking brake. I don't know if it is normal, but every MB I have had does this, and I know because every MB I have had will eventually develop some strange noise that requires diagnosis with the back wheels off the ground, one was a rear bearing, another was a driveshaft out of balance, etc, etc. I wouldn't worry about it, but it is strange.
Brian

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Brian W. Heitman

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