HP Z2 SFF G5 - PSU fan spins at max when computer is off

[boney2]

I recently bought a used HP Z2 SFF G5.
As soon as I connect it to the mains, the PSU fan starts spinning at its max. It only stops when I press the power button and the computer starts booting. While I'm using the computer, it spins at a normal speed and everything is fine. All basic hardware tests pass without fault. However, after shutting down, the fan starts blowing at its max again and I need to disconnect the mains power to get it to stop.
The PSU never feels hot.

I'm assuming this is not normal. Does anybody have any suggestions?

I have tried randomly disabling and enabling various bios settings but nothing has fixed the issue so far.

[DGroves]

some things to check

what cpu is installed? the higher rated ones produce more heat and will usually have a optional case cooling fan and possibly a shroud installed that the lower wattage cpu's do not need

is the case fan spinning? (if installed) 

also check the cpu fan and power supply  fan(s) are not spinning slow when powering on system

the system sets the  fan speed by checking the integrated temp sensor which for this model  is part of your power supply as i recall

if still under HP warranty have them check the system and replace any suspect parts

[boney2]

It's a Xeon W-1250 but this is not a heat issue; the PSU fan spins at its max from the moment the system is connected to the mains, before it has even had a chance to generate any heat.

There is no case fan; only a CPU fan and a PSU fan. Both spin normally once the system has been powered on. It's only when the system is off (but still connected to the mains) that the PSU fan alone spins at its maximum.
It's still under "factory warranty" (service type: HP HW Maintenance Onsite Support.) I'm not entirely sure what that means.

[SDH]

Boney2,

That sure sounds like premature activation.

From what you say it sounds like you have not yet even pushed the power-on button and yet the workstation gets activated upon plug-in. Sounds like potentially a short in the power supply or the motherboard. I'd personally try a new-to-you used replacement power supply first.

Good luck...

[boney2]

Yes, I am certainly of the opinion that it is a hardware fault.

I'm hoping the warranty will cover the repair despite me not being the original owner. HP repaired my laptop under warranty a few years ago. I wasn't the original owner of that either - and one of the reasons I've continued to use HP was because I was impressed with their service back then. Hopefully things have not changed.