Issue happens on Citrix ADC MPX 5550 and MPX & SDX 8005...8800 platforms.
Customer enabled SNMP Alarm for fan speed low alarm according to ADC document:
https://docs.citrix.com/en-us/citrix-hardware-platforms/mpx/netscaler-hardware-system-health-attributes.html#health-attributes-for-mpx-55508005115001750017550-platforms
Where edoc suggest MPX 5550's FAN speed should be >5500 RPM.

Alarm: set snmp alarm FAN-SPEED-LOW -thresholdValue 5500 -severity Critical
After upgrade several MPX 5550 boxes, customer starts to see this alarm generating.

Workaround：
For MPX 5550 devices, we have an internal known issue under discussion to determine the proper Fan speed RPM health attribute.
As a workaround for now, we suggest customer to use CPU temperature as the key monitoring indicator.
CPU temperature should stay below 80C.
Use SNMP Alarm CPU-TEMPERATURE-HIGH
set snmp alarmCPU-TEMPERATURE-HIGH -thresholdValue 80 -severity Critical
https://developer-docs.citrix.com/projects/citrix-adc-command-reference/en/latest/snmp/snmp-alarm/#enable-snmp-alarm

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Problem Cause

The health attribute documentation we pointed to used data collected in the controlled environment of Citrix labs, not taking into account our datacenter products are spec'd 0C to 40C (newer platforms are spec'd 0C to 45C, tested -5C to 50C.

So Fan speed below 5500 RPM is not an issue, Citrix Engineering is still discussing how to update the document to provide more qualified Fan speed attribute.

The Fan Speed health attribute is still under discussion, FAN RPM<5500 is not an issue. Consider using CPU-HIGH-TEMPERATURE SNMP-Alarm as workaround