This will sound [ Counter Intuitive ] but trust me it works.
I have an ML110 G6 with a Xeon 3440 CPU, and 16 GB of RAM.
I thought the [ Rear Fan ] was loud and went looking for a solution. I tried setting a threshold in the IPMI BMC controller using one of the network IPMI tools. But that did not work. The IPMI thresholds are only for reporting or intiaiting actions based on crossing the thresholds, not for controlling the Fan speed.
I now believe it is the BIOS and something called the [ Fan Curve ] in the BIOS version that controls how it ramps up and down and where it stablizes based on Ambient room temperature (there is a sensor at the front under the Bezel) or whether there are additional Option Cards installed.
What [ Works ] for me is to go into the BIOS at boot POST time, you must press [ F10 ] then go into the [Advanced] column menu, then go into the IPMI submenu.
[Main]->[+Advanced+]->[+IPMI+]
There is a option called the [Blow-Out feature].
If you read about it it says it will [ Run the Fans at High Speed ] presumably to "Blow-Out" the dust in the chassis and then settle down to a lower RPM if -->[+Enabled+]. I would guess if there is a Management agent installed on the Operating system it could then take up the task of monitoring sensors and ramping the Fans speed up and down appropriately.
I thought "Disable" would lead to a Quieter start up and run, It Does Not
If "Disabled" the Fans will run at "High Speed" until told otherwise by an HP Management Agent in a supported version of a supported operating system. If you run an unsupported Operating system without a Mangement Agent, or do not install an HP Management Agent installed on a supported Operating system. The Fans will start and run at "High Speed" all of the time.
I also suspect if anything like "antivirus" or a million other accidental things interferes or prevents the Management Agent from controlling the Fans, the Fans will always run at "High Speed" all of the time.
The safe option is to always leave the "Blow-out feature" [+Enabled+] its the only real control you have over the Fans without some special software agent that knows how to control them.
The Rear Fan still runs high, around 2000 RPM versus 4000 RPM, but it is audibly much more tolerable.
The Rear Fan is a standard 90 mm x 25 mm with a Tach sensor and PWM control, and a locked Rotor Sensor.
The IPMI thresholds seem to suggest anything below 900 RPM would log an event in the SEL.
And a locked Rotor signal (+V ) would keep the system from starting. This is to prevent thermal damage should the fan be unable to turn, for example if something gets jammed in it from the outside.
The pin out is also non-compliant with the Intel specs for Fan pinouts
A few people on the Internet have mapped the pin outs and they come down to approximately the same number:
Tach
PWM
Locked (Rotor) Sensor
+12V
Grd
It is the Locked Rotor Sensor that I suspect keeps people from successfully swapping out the Fans for quiter options.
A Warranty replacement Fan can be purchased, but it will conform to the same audible characteristics already observed.
A few people on the Internet have reported success at wiring a custom crossover connector between the motherboard and a new Fan with lower audible accoustic properties, similar to the Nocturna line of Fans. Which would most likely violate your warranty.
But making sure the "Blow-out feature" is enabled has made it possible for me to update the BIOS and the LO-100 firmware up to the latest and still have the fans loud, but tolerable. Its a far better option than letting them run at Full High Speed for extended periods of time.
Note: Resetting the BIOS, removing the battery, ect.. will I assume return the BIOS to its default settings, which will also reset the "Blow-out feature" to the [+Enabled+] position. After listening to the Fans whine at their top speed for a while in frustration, "any" relief can sound quite major.
After thinking about this for a while it makes a kind of good sense.
The default position leaves the system in a "relatively" quieter state if there is nothing that will be subsequently starting up to control the Fans. However the best control (and possibly the lowest RPMs) would be attained if the Management agent comes online and appropriately throttles the Fan speed back as much as possible.
The LO-100/BMC controlling the fans appears to be made by ServerEngines Pilot II by a company in San Jose, CA which was recently acquired by Emulex to combine their Converged Network adapter tech with their Fiberchannel adapter technology. So there may not be a lot of after market information available to write a custom control program.
The best examples of that would probably be the finished Management Agents provided by HP.
I'll go out on a limb and [guess] that the HP Management agents "might" expose an SNMP OID or WBEM attribute in the CIM which could be used to [set] the Fan motor PWM or equivalent RPM, to grant greater control over the Fan speed. But that also assumes you have the appropriate "agent" or "provider" and know where to look.
HP makes some MiB schemas available for their management tools, like SIM -Systems Insight Manager, there might be something that could be "found" walking the MiB with snmpwalk or browsing the CIM that would expose a controllable parameter.. but it does require you to have the HP provided software already installed.