There’s no trapped air. The impeller is clean and nothing is restricting the flow. The O ring is fine, they come lubricated.
I actually ran the filter without the basket and media to check it worked and it made a humming noise then. I hoped it would be quieter in use with media in it. Even though it was the wrong kind of sound I thought it may be vibration going through the cabinet but it’s still noisy if I hold it in my hands. I can reduce the noise by maybe ten percent if I put hand pressure on the top of the casing.
There is very little vibration.
I had the exact same model many years ago and it was virtually silent in use.