when working perfectly the fan noise is never louder than the level of a noisy PC
That's exactly what I was used to with my former Z2100. One could here the fan making a constant sound, louder than my (silent PC), but not loud in general terms, and I got used to it in a short period of time.
With the Z3200 however, not only the formatter fan is running, but also the additional fan on the left side of the printer (seen from the printers front) which is meant to produce the vacuum to keep away ink aerosols from the rest of the interior when printing. This vacuum fan is the one making the noise, in operating mode and
in standby mode. This fan and its counterpart on the right were only running on the Z2100 when it was in operating mode. As soon as it went back to standy both of these fans were quiet.
My understanding is that either something is wrong with this vacuum fan or it is not slowed down/powered off when the printer is in standy, because of something wrong with the electronic assembly.
Might be a case for the HP warranty.