Sorry I don't have a picture of it, but there is a fixed metal shaft that the impeller sits on. Once you pull the impeller out it is visible. Place a washer on the shaft and insert the impeller. Push in on the impeller gently and if it hits the face of the pump, remove it and add another washer. The magnetic field tends to hold it in one position until the pump runs and other forces overcome it causing the impeller to move in and out. Thats where I felt the rattling noise was being caused by, and it contributed to the vibration.
In regards to what the hole did to solve the problem, I honestly don't know. One would think if the impeller lifts off the shaft while it spins, coolant would flow into that space since there is clearance between the shaft and impeller bore. It is hard to believe a small air pocket would remain between the blind hole of the impeller and metal shaft. When I take the pumps apart, I found traces of a lubricant on the shaft. I tired putting some water resistant grease in the impeller bore and reassembled it. The pump would be quiet for a few days, but it would eventually wash out and the rattling would return. Once I added the hole, they are quiet.
If you attempt to take the pump apart, be aware that the case of the pump is very thin where the lock tabs of the end cap are. To get the end cap out, you can use one of them rubberized cloths that you might have in a kitchen drawer to assist in getting jar covers off. Wrap it around the inlet and twist to remove. You can also cut a piece of hose to wrap around the aluminum inlet and use some pliars. That way the aluminum anodizing won't get damaged. As you reassemble it, push in the end cap while turning so it follows the slope moulded in the pump casing. That way the three ears on the end cap won't stress the thin plastic where the lock tabs rest. If you look at the 12 oclock position of the lower picture in my first post, you can see where I nerfed the plastic with the end cap lock tab.
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