Fan speed does not change (but nvidia-smi says it does)

Following other posts in this forum I have tried to change the fan speed doing
sudo nvidia-settings --display :0 -a GPUFanControlState=1 -a GPUTargetFanSpeed=60
(I have also tried without the --display)
The response is

  Attribute 'GPUFanControlState' ([gpu:0]) assigned value 1.
  Attribute 'GPUTargetFanSpeed' ([fan:0]) assigned value 60.
  Attribute 'GPUTargetFanSpeed' ([fan:1]) assigned value 60.

So it seems the change has been applied. Indeed from nvidia-smi I do see the fan speed value decrease to 60%. I can try also lower values down to 30%. For values below 30 I get an error that reminds that one can set values only between 30 and 100.

My problem is that no matter the value that I set the amount of noise from the computer is literally unchanged. I was expecting that with 30% the system would be a bit quiter than for 100%. So start to doubt that nvidia-smi is telling me the truth.

So I am wondering if I am using a no longer supported solution. For instance I see that in the past lots of people has suggested setting Coolbits to 4, as this was enabling fan control in earlier versions of the driver, but I see that for 530.41.03 NVIDIA Accelerated Linux Graphics Driver README and Installation Guide the Option "Coolbits" "integer" does not list anymore bit n.2 (related to fan setting).

Am I following wrong instructions to set the fan speed? What should I do to set it to something more quiet?

I have tried the graphical tool nvidia-settings as root and the behaviour is the same. I can set the fan speed to lower precentage and the reported RPM go down to 800rpm for 30% instead of 3000rpm for 100%, but the amount of noise is really unchanged (measured with my phone it keeps the same sound pressure and frequency spectrum, quite flat from 0 to 8 KHz apparently).

EDIT: The 8 KHZ max frequency of the noise triggered my curiosity. The noise from a 3000rpm fan cannot be so high frequency … Indeed I figured that it was my chassis fans from Dell T640 that spinned so loud. This was in response to installing a PCI-e card for which the Bios has not a predefined “thermal profile”, hence it maxes out the fans to ensure appropriate cooling. So, I can confirm the nvidia-smi report correct speeds. and the syntax highlighted above does indeed make the fan spin as per user setting. Other DELL users may find useful to install the appropriate version of RACADM and control the response to PCI-e cards by RACADM giving sudo /opt/dell/srvadmin/bin/idracadm7 set System.PCIESlotLFM.8.LFMMode 2 where 8 was the PCI slot I used for my card.

Thanks for your help