Same thing is happening with me after replacing the fan. This is clearly a firmware issue imo
I have a brand new Bambu X1C printer that I received a bit more than a week ago.
When I powered on the printer for the first time and connected it to wifi, it said it had a firmware update. Did the update, and did some prints.
Was quite positively pleased that the printer was not as loud as my previous decade old Ultimaker 2 workhorse.
I keep the printer in a separate room that is connected to my office room, with a closed door in between.
Yesterday I wanted to dry some PVA filament, but noticed that X1C “Dry Filament” option did not have an option to choose to dry PVA. Browsing the menus, I realized there was another firmware update available (previous update having taken place just a week ago), so I decided to upgrade to that to see if PVA drying would now be available in the menu.
Updated firmware, and reran the whole calibration sequence. (PVA drying is not listed in the menu, so I dried it as PA. Hopefully close enough)
However, after that update, I notice that the printer is all of a sudden noticeably louder than before the latest firmware update. I don’t have the version numbers since I didn’t think to make note of them, but there is definitely a difference.
Bambu Studio lists “Part fan 100%”, “Aux fan 80%”, and “Chamber fan 90%” in my current print. Reducing the fans to 50% does help a lot with the noise, although I don’t think I’ll do that as a solution, since it could affect reliability.
I wish these fans were quiet - would love to do a Noctua treatment here, though the fan curves and functional parameters would probably be all over the place after that, so not quite sure that would make sense. I wish manufacturers would be serious about choosing quiet fans, those would help the comfort of maker spaces by quite a bit.
I have the same issue. The fans are not even at static speed, they fluxuate and makes them sound even more weird. I have manually reduced the speed (but it changes after every filament change). The printer is sitting next to me. Before the update the noise was bareable, now it’s not. Bambulab, something is messed up.
I’m also starting to experience this now — it never happened to me before.
It started just a few days ago, and now it happens in every print.
I contacted support, but they told me it’s normal behaviour for the chamber fan.
Still, it doesn’t seem normal to be fluctuating this much and this fast…
Especially since it used to stay steady for me.
Really odd that it’s suddenly happening now.
from what I gathered back when I replaced mine is that the temp sensor inside the fan can become faulty. since I bought a new fan and installed it, mine played up for a while but then settled in to working okay. still dont know if it was fixed by firmware or the control unit just needed time to realise I’d put a new fan in
just a note of caution if you replace yours - the mounting points of the fan are WEAK. like really brittle. 2 of mine snapped when taking the old fan off. had to use gorilla glue to fix them up
The thing is my X1C is basically new (~1-2 months old) and less than 100 hours of printing. So it also sounds very strange that the fan became faulty (but it can be of course).
interesting 
so what print bed / material you using?
I’m using the textured PEI and Bambu Lab’s PLA matte.
only thing I can think of is if the weather is really warm/humid currently, maybe it’s ramping the fan up?
I would understand that if the fan was just running higher than usual,
but going up and down between 40–80% in less than a minute — that behaviour I don’t understand.
How does the change in speed reflect so quickly on the sensor that it has to adjust again just seconds later?
I had a similar situation. The chamber fan either rev’d at constant high speed or kept changing revs, also at rather high levels. It happened to me right after updating my X1C firmware from 1.7 to 1.10, which I immediately regretted and reminded me again to not fix what ain’t broken.
I tinkered with the P3 (chamber fan) speeds in the filament start G-code settings to no avail. Lowering fan speed on printer or Bambu Studio was effective but reset once the printer advanced to the next stage, like flow calibration > first layer > print. In any of those transitions the printer got take off clearance and went full throttle.
I swapped out the carbon filter to a new one as an act of desperation, knowing this won’t fix anything as the one in place can’t be clogged with 170hrs of PLA. One print later it was confirmed desperation.
Next thing I thought a factory reset could help. Ran through setup after reset, vibration compensation and noise calibration. After the first print the constant speed change and revving up to 100% is gone.
PS: Rather late to the party but if my response could fix a single persons chamber fan problem, it’s worth the space.
I have this happened for so long that I kinda got used to it but still annoys me 
And I contacted BambuLab and they just said it was normal and ended up ignoring me. I have done a factory reset but no success, the issue still happens.
Is anyone experiencing this as well? Any solutions? Is it a faulty fan or sensor or something?