The whole point of this very long post is to maybe help solve some of the mysteries we have had on the ps4. I'll start by saying yes, I was one of those guys (keyword WAS dont crucify me) after having ny launch ps4 blast into outer space would exchange their PS4 at Gamestop using their product replacement plan. Also, I was very aware of dust buildup and kept the area the PS4 was placed open and as dust free as possible. More on this later, but i also did extensive research on what how different parts were actually put in different ps4's. So, using Sony I sent that first one in for repair got back an even worse cigarrette smelling one. Sold that one, bought 1100 model from gamestop. Quality was BAD on several units ranging from bad disc drive to bad HDD to yes very loud fan noise. I fell into the "my unit is totally quiet works perfectly fine etc." Plus i think my luck was just shit. Its not like i immediately turned around and exchanged the units within a day or so i gave each one plenty of time. Eventually settled on a unit that was fine and tolerably loud. Then, the 1200 model came out with new internals and supposed quieter fan. Traded for the Destiny Taken King model and struck gold on the first one. It was quiet on everything including the big offenders of the time like TLOU and bloodborne. Super happy. THEN, the Pro hit and I had just got a Samsung Ks8500 4k so i traded for the Pro. Again, struck gold, first unit was quiet on everything including Pro patched games like FF15 and Resident Evil 7 which only made the fan kick up just a little even in VR I'd say 3/10. Non pro patched games were a 1/10 on the noise scale. Story doesnt end though, my pro as of a couple weeks ago went kaput. I suspect more because of a storm that came through over the PS4 power supply going. So, I go and exchange it thinking by now shouldnt be any issues still. Wrong, got one that immediately got very loud and hot. Exchanged that one and got one that I would even argue to say is better than the first.
Now with the story out of the way and back to what I researched with each ps4 i found several differences between each unit I got even though some were of the same model series (1000, 1100, 1200, Pro).
1. Internal hard drives were different. Some models had an HGST hard drive, some had a Samsung Seagate Spinpoint, and some had Toshiba hard drives in them. The Toshiba drive was the one that was DOA.
2. Blu Ray drive noise also differed for some reason. Some units were quiet no matter what disc and some rattled like hell no matter what. I assume it may be how they were mounted.
3. Controllers were even different. Some rumbled much harder than others but I paid the least attention to that.
4. The most important aspect and most widely known issue.. FAN NOISE. This one aspect had a few very distinct differences on noise and behavior. From deeper research i found the maker of the fan differed between Delta and Nidec. If you had a delta fan you most likely heard a sort of whistling varying pitch noise. If you had a Nidec fan you more likely heard just the whoosh of air being expelled from the unit and these fans were generally quieter. Just by listening to how that sound is will pretty much tell you which fan you have in your PS4 and that includes the Pro. The other thing I noticed which ranged from all models was the actual behavior of the fan itself. The "louder" fans seemed to vary in speed greatly depending on load and from one minute to the next go from really loud to quiet. Closing a game would immediately send a very loud fan to dead silent in seconds. Any tech savvy person knows that is bad for chips as they dont cool down immediately like that. The "quieter" fans bahaved completely different. For an initial mayne 30 minutes the fan would vary in speed but only slightly and not dependant on load also not getting near as loud. I attribute this to the nidec fan as well. After maybe 30 minutes the fan would reach a constant speed and never change either higher or lower while playing. Upon quitting out of the game back tp the dashboard, the fan would continue to spin at the same speed for roughly about 5 minutes until it would go at minimum speed. This is how I believe it should behave.
That's from my experience all i know and hope it helps you guys and let me know if anything Ive said here is similar or different to your experiences.