Scenario: I have a Thermaltake case, all tempered glass (View 31), and a CoolerMaster Hyper 212 RGB Black cpu fan on my cpu (i5 9600k on an Aorus B360 gaming 3 wifi board). It had not produced any significant issues until the following occured: I had the system off, and was working on the system beside. It was hot in the room and I didn't want to turn on my AC yet, so I took my Honeywell desk fan and popped it up on top of the tower. Next time I booted up the tower there was a horrendous rattling/humming/droning/thrumming, very loud, apparently coming from the cpu fan exclusively and very distinctly, the only fan seeming to make any noise in the whole chassis (removed both the glass panels to be sure there were no other culprits). Of course I took the desk fan off the tower before that point and turned it off (I had actually removed the desk fan from on top of it prior to removing the glass panels btw, would have been difficult to manage so otherwise). The rattling persisted, it was very ugly to hear. I turned off the system for a few seconds to inspect the fan. Fan seemed fine upon giving it a spin with my finger and checking to see if it was still fastened securely with the thumbscrews, all screws seem tight that I can see (but I didn't build this system and don't actually know how to unmount the fan from the cpu at this time, though I haven't looked much into it). Turned on the system and it was normal again (this system has always been very quiet compared to my others). I thought this some kind of bizarre phenomena and decided just to keep an eye on the system. Couple of days later, now having decided it was likely a bad idea to sit the desk fan on top of the tower due to concerns about vibration. Desk fan had been running overnight, about a foot from the tower now, this time on top of a stack of books on top of my printer (which has not been in use lately), which sits on the same desk. Turned off the desk fan earlier that morning. Later, turned on the system, not even suspecting anything would come of it, having assumed the odd phenomena was a one-off, but... SAME THING. Horrible unhealthy rattling/humming/droning/thrumming sound, most certainly emanating from the cpu fan specifically. I even left the system on for a bit to see if it would subside, but nothing doing, persisted whole time. System appeared to work fine otherwise. Turned it off a few seconds. Turned it back on. Normal again, quiet as a baby. WHY? Just WHY? SOOOO confused. I'm not an expert in any science, but is this some kind of electromagnetic thing going on between the desk fan while it's running beside the cpu fan while it's off? Some kind of build up of electromagnetic interference in the cpu fan from the desk fan running nearby? For the record, the desk fan is plugged into a power bar, all peripherals and the tower itself are plugged into a Cyberpower UPS with AVR. At first I had suspected the vibration of the desk fan may have been the cause, having originally set the desk fan on top of the chassis, but the desk fan in this second instance was on top of a stack of books sitting on top of the printer, which itself is on the same relatively wobbly cheap press-wood desk, which yielded no vibration to the touch while the desk fan was running, more or less convincing me vibration had not been the cause as originally suspected. Of course, my next test will be to turn off the PC and not run the desktop fan this time, then turn the PC back on after a while to see if another cold boot produces the same effect without the desk fan having been running. But this whole thing makes me feel a little crazy. I've never seen any effect like it before. Is there some science that can explain what's been happening here? I don't suspect the system has been damaged at all, but it certainly sounds unhealthy as anything when this occurs. Have any of you ever seen anything like this? Any help/insight would be greatly appreciated. Note: I have indeed also been trying turning the desk fan on while the CPU fan is running. No effect. Still normal. It's been running perfectly quiet and normal for hours now atm of this post. It had also done so after the first time the phenomena occurred, for the record.