There is a pulsing hum regardless of if its idle or not, it seems to be the HDD but kinda sounds like its coming from the power button which would make no sense seeming as there are only cables going up there.
Is it normal for HDD's to make a pulsing hum? I don't think its coming from the HDD directly but that its vibrating the case but I'm not experienced with this.
Since it's booted up today the humming has calmed unless you put your head near the computer but now there is a higher pitched sound, its still quiet but its almost like tinnitus or crickets at night which also seems to be coming from the same place.
I guess I'm a tad sensitive to sounds I have to sit around all day at least when it comes to my computer but its a lot less quiet than coil whine, if I had that coil whine it would drive me insane.
Out of all the parts in my computer I never hear the cpu or gpu but the HDD is constantly humming/pulsing and crackles when I do stuff related to it. Maybe its just because I'm paranoid and new to computers but if a part broke I wouldn't be able to replace it let alone if it took down something else in the computer with it.
It could still be a fan. If you shop for case fans you'll notice some have rubber on the corners near the holes where the screws go. Those are to help prevent movement and vibration from the fan from transferring to the case and making the case or fan rattle.
The pulsing sound that fans can make is cause by wobble. It is much like a top spinning, even though it is spinning fast and there is a lot of centrifugal force, there is still a tiny bit of wobble. In a fan this can result in a pulsating change in pitch and tone.
Have you accepted Raspberyl as your loli and savior?