I would suggest to open a case with either ASCs or PaloAlto TAC if you have access to.
As we know that FANs are there to cool down the device CHIPs and processors running into and may be something inside is heating up and that's why FANs are running at high speed which is good but not always.
You cannot control the speed but you can check for any alarms (critical severity system logs), LED status, and system environmentals from CLI.
Command: show system environmentals.
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