Water Heard in Tub but NOT Seen in Tub:
Water heard in the washer but not seen when the tub is moved is from the balance ring in the washer. The balance ring is a fluid filled ring at the top of the tub.
As the load shifts during wash, the load may become more concentrated on one side of the basket. The fluid in the balance ring shifts to the opposite side of the load to help offset the weight of the load and keep the rotation of the basket stable.
If you move your washer, you may also hear the fluid inside the balance ring.
The balance ring keeps the load balanced and the basket running smooth which allows higher spin speeds.