Since it is quiet and then after a time begins to be noisy, this may not help but a thought for the next time you have it open. I'm not up on what type impeller shaft the Sun-Sun uses. It might be worth a look at the shaft. Laying the shaft down on a flat surface with a bright light on the far side, roll the shaft slowly and look for light leaking through under the shaft. Metal shafts are prone to having a small amount of wear or bent very slightly. The ceramic shafts like Eheim are fragile if we drop them but wear much better and don't wear as fast or bend. On an older filter, a bit of sand might have gotten in the space between the shaft and impeller but not likely on new. Trying a new shaft and impeller would be one way to test for this noise but quite expensive.
Another item which I find may cause me trouble is when the media is too dense or a bit clogged so that the water doesn't flow through well. I find this can cause the outgoing water to be pumped out faster than the water comes in and it makes something of a "cavitation" type noise. Perhaps removing some of the media trays for a test when the filter is making noise would help spot this? Not suggesting removing the media and leaving it out but just remove, reinstall the filter and test for the noise. Then reinstall or adjust the media if it tells you anything. Some way, too much media makes a couple of my canisters suck in air, even though I can't find a leak under other conditions. I first noticed this when I changed media and had trouble priming a filter which had never given me trouble before.
Growing plants and fish teaches patience if nothing else??