Depending on the condition of the environment where the device is installed, fan speed might change frequently or run at high speed for most of the running time. If the change in fan speed seems not related to the surrounding environment, you may take several steps to identify the issue.

The customer experienced periodic spikes in fan speed and identified that the noise was from a specific PEM module. 
The following steps were taken to narrow down the issue further. 

In QFX10008, fan speed for each fan in fan trays and PEM modules can be monitored by operational commands. 
You may use the following commands to monitor the temperature of each component within the chassis, fan speeds in RPM, and power usage. 

show chassis environment
show chassis fan
show chassis environment fan
show chassis power
show chassis environment pem

You can use these commands to identify which fan is running at high RPM and check if the temperature of any component is unusually high. 

Once you narrow down the problem to a specific FRU, you need to take further troubleshooting steps to distinguish whether the issue is due to the FRU fault.

You may replace the FRU in other slots and see if the issue is seen at the moved slots.
If the issue is seen on specific FRU modules, regardless of the installed slots, you need to open a case and request an RMA.

If the issue is not specific to a certain module but instead seen on a specific slot only, you may need to open a case for further troubleshooting.