This article explains why the fan trays on EX switches seem noisy after upgrading Junos OS 12.3.

After upgrading Junos OS 12.3, fan trays on EX switches seem noisy, but the output of the show chassis environment command reveals that the fans are spinning at normal speed, so there are no alarms on the devices.

The fans seem noisy because they are running faster.

The fans are running faster because of the changes made to the temperature threshold to protect the hardware from failing at higher temperatures.

A check was added in Junos OS 12.3R6 to warn users that the Uboot version is unable to accommodate the new feature/faster fans:

Alarm "Firmware version not latest for FPC."

2020-06-19: Article reviewed for accuracy; no changes required.