Something else that may be helpful/relevant was that to reduce the amount of dust getting into my system, I took a thin piece of sponge (about a centimetre thick) and taped it over the air vents on the side of the case. Internal temps have been unaffected.
You could play around with enabling/adding a chassis fan in another location on the case while blocking the top one?
One other thing to bear in mind about reducing noise levels is the position of the case in the environment. My case is partially hidden from my 'sitting at desk' view and so the noise levels will be affected by that, but also the door to the room is about 1.5m away from the case; leaving the door open means that the computer sounds less noisy, and the door closed equals more noise presumably because the sound is reflected back to my position.
In most theatres/cinemas, cushioning material such as thick curtains or something like wall carpeting helps absorb sound, whereas sitting your computer in the corner of the room means that the sound from the computer is reflected off the walls nearest to it and likely back towards you.