My experience is the same as Doogy. I have SunSun 304B's are they are all extremely silent. I too have to look at the surface agitation to tell if they are running. Even from a foot away I can't tell they are running. No mechanical vibration or electrical buzz or any hum noises at all. I have good hearing as well, though I am not overly annoyed by slight noise like some people, but still my canisters are silent.
Only time they make noise is if they have air trapped inside. Tip the canister front, back, side to side to let air escape. Or maybe something is contacting the impeller (I've never had this happen). Or maybe the canister is touching something and sending vibrations out.
I've tried finding example of the lack of noise mine make on youtube vids, but they all seem to have a faint hum, which mine don't have. Not sure why is that. Back when I first bought my canisters over a couple years ago, there was a video that had a dead silent SunSun just like mine, but I didn't manage to find it with the couple quick searches I did. I don't place mine on any dampening material (not even a towel) or anything. Mine are even under the tanks on open stands (just framing, not enclosed). Some are sitting on wood on the room floor or on a plywood sheet on a metal stand. I've tried mine jampacked with media, little to no media, under tank, even level head height and still no noise. And yep, I have tested it in a dead silent room, even at night, even with nothing, but the canister running.
How bad is the hum? Is it like typical noises as heard in youtube videos? If so, then maybe some just do for whatever reason have that hum and returning/exchanging it might not help. But if it is obviously louder than the vids, maybe there is some slight quality control issues that is causing the noise. Try placing a towel under the canister if you want to see if that helps.