2003 Buick coolant loss

Every week I have to top off my radiator. About 6 ounces. Cannot see any leaks anywhere. oocasionally there is a noise coming from the area of the water pump. Kind of sounds like a deisel engine. Like shaking a coffee can full of nuts and bolts. Again, this only occurs once in a while . Any help is greatly appreciated

When the coolant gets hot (about 200F degrees plus) it expands. When it expands, it has greater volume. This greater volume pushes coolant past the radiator cap and into the coolant overflow bottle. When the engine cools, the coolant un-expands and its volume decreases. This creates a little vacuum in the radiator. The radiator will actually have an unfilled space. The vacuum pulls coolant back into the radiator from the coolant overflow bottle.
There may be a problem with the vacuum hose from the radiator to the overflow bottle, or the bottle, itself. The vacuum may not be happening if air is getting sucked into the coolant system. Air could be sucked in anywhere, including around, or through, the water pump.
Have the coolant system pressure tested. Change the radiator cap. Have the overflow bottle and vacuum hose inspected.