Fluval U2 Venturi

The first thing is that you don't need to use a venturi. All it does is send bubbles shooting across the tank with the water outflow, then as the flow loses speed they float to the surface. The water outflow by itself is enough to keep the water turned over.
And the manual does say in the section Filter Preparation
"8. If using the venturi aeration system, place it on top of the nozzle (g)" (my italics and bold)
Which means you don't have to use it. My Eheim filters (Aquaballs and Biopowers) also have an optional venturi. I tried it with the first filter I bought and the bubble noise annoyed my husband so I've never used them since.

Looking at a very enlarged image in the manual on my laptop, the venturi has a little circular arrow on the flat top. Presumably you turn that to increase or decrease the amount of bubbles by regulating the amount of air it sucks in. Have you tried turning it? The top part will need to be out of the water for it to suck air in.

Mine complains about the humming as well. When the 125 litre bracer bar broke and I suggested selling the piano in the dining room and getting a bigger tank for in there, he jumped at it. Anything to remove the noise from the lounge.