Testing a CRS312-4C+8XG destined for office environment and I have to say the noise level is decent at ~4k rpm (as reported by Winbox with ROS 6.49).
The problem is the fans will stop spinning under a certain temperature (~50 Celsius CPU and ~ 55 Celsius PHY), the temperature will quickly rise above 60 Celsius, fans will start spinning again and the cycle will repeat non-stop (ambient 22 Celsius).
This variation in sound is the annoying part.
At the lowest speed (as far as I can tell) of 3800-4000 rpm, the fans are doing a very good job at keeping both CPU and PHY at around 50 Celsius, while being decently quiet.
If future versions of software would allow the user to disable this zero rpm behaviour would be nice.
As a side note, if future versions of hardware would be fitted with larger heatsinks, would be extra helpful.
A quick 16x speedup video: https://streamable.com/lkjn93