Either you have too much water in it, or your water separator is missing teeth. The water separator is the thing that spins in the center of the motor where your water basin attaches to it. It looks like a cone shaped fan looking thing. It should be cleaned after every use.
Use a your Rainbow separator cleaning brush, or a tooth brush, if you do not have the Rainbow one. They are just like tooth brushes.
You can get manuals either here,
Never store your Rainbow canister on your water basin.
You fill the water basin to the top of the knub thing that is inside of it.
If it is a standard water basin, it holds two quarts, of water. If it is the larger one, it holds 4 quarts of water.
If your vacuum begins shaking or swaying during usage, that meens that you need to poor out the dirty water, and start over with fresh water, because the water level will rise as it gets dirtiet.
If you have any more questions, let me know.
I am a Rainbow vacuum expert.