I recently bought 3 of AQ20 filters, so I have a bit of experience with this kinda thing. My first one was perfectly silent which is what made me want to get 2 more for other tanks. The other two were really noisy, and keeping the lid on made it worse.
After some experimenting I'm pretty sure that it has everything to do with getting all of the air out of the filter when it is first starting up. When I first put an AQ in my tank, I use a cup to scoop some water into the filter so that it doesn't run dry. Then I plug it in while set to maximum flow. It takes it a minute to suck the water up the tube, but you can speed it up by moving the lever back and forth from max to min. But it was this trick that seemed to cause it to be more noisy. Just leave it set to max while it slowly gets going, it might take longer but in my experience results in much more silent operation.
I'm really curious if anyone else has had a similar experience...?