There is a Teflon washer between the head and body. Over time that washer will flatten and cause the pulsation you are seeing. I'd recommend replacing it and see if the pulsation goes away. My bet is that it will. On my two brushes that have that washer I always loosen the head before storing them to help prevent that from happening.
It is part number 50-055 in This Exploded View of a 200, right between the head and body.