The movement is caused by a leaking manual valve in the valve body, or a leaking/sticking valve in the valve body (unless you are saying that the car is moving more than 6 inches, in which case likely your linkage is out of alignment or worse). Some cars will start doing this before
VB failure due to excessive filter service intervals ("paper" filters that MB sells will blow out after so much filtering, allowing unfiltered fluids to travel through the valve body), but yours is likely just normal wear and is nothing to be concerned about.