I don't see the option to add to PWM value in my 3.60 BIOS. How do you navigate in BIOS to that?
I can't hear my Z400 with the HP fans as I have tuned them, but will add in a front HP PWM cooling fan to see if that drops the HP PWM chipset fan speed just by that act. As noted that shows up in the BIOS listed as the "Memory" fan.
That is a 40x40x20mm official HP PWM chipset fan that is used in the xw6600 and the Z600, and it would be nice to have that run at about 2000 or 1500 RPM given that HP never had an official chipset fan on the Z400's chipset heatsink, to my knowledge.
Keep us posted..... your work will be appreciated.