I have had this for about 8 to 10 weeks. Has been working great except for a few “filament stuck in extruder” errors. All of a sudden today while printing PLA the rear exhaust fan started making a loud noise. I can’t tell yet if it is a bearing or if it is spinning abnormally fast. I would appreciate any suggestions
How often do you clean out the chamber of the base of your printer? It gets awfully dirty down there.
If you have a sudden noise in any fan, there are usually two dominant causes. A) fan bearing is defective. Not likely in this case since Bambu fans are non-bearing fans. That means when they fail, they just stop working, they generally don’t make noise. B) Contaminant that has clogged in the turbine blade. Most likely cause. That’s why the question of how often do you clean the bottom of the chamber.
Either cause is easy to fix but the second cause is the easiest. Just disassemble the fan and look for debris, if found, remove said debris and reassemble.
In the case of a defective bearing, the fan will have to be replaced. Keep in mind that the Bambu power system utilizes 24Vdc whereas most PC fans are 12Vdc.
Which reminds me of a story last summer where my friend asked me to repair his Son’s overheating laptop.
His Son Loves sunflower seeds. Laptop? Not so much. Once removed from the fan “Viola” overheating problem solved. 
I had the same problem yesterday. All of a sudden the chamber fan starts spinning with 100% when the print starts and that thing can get very loud… I had to manually switch it of through the mobile app. I checked the slicer settings and it wasn’t activated anywhere. I did three prints yesterday and had this issue with all of them. Had no time to do any further throubleshooting. Maybe your printer is doing the same thing?
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Probably a normal thing. Remember, the chamber fan speed is controlled by the bed temp. If you print over 55C bed temp, they know the chamber will get very warm so they spin up the fan to 100% for PLA. At 45-55C, I think its around 70%. Under 45C, its running around 50-60% (can’t remember). But you can check the exact speeds in the slicer’s Start GCode tab.
Have you found the cause of your problems? I just got the same issue as the one you described