What OS are you running?
Regardless of the OS, you should install the HP drivers and management agents appropriate for the OS. If using an unsupported OS, you may not be able to do this (but you should be using a supported OS)
Fan speed and thermal monitoring is controlled by the ILO. Ensure that the ILO itself is up-to-date (version 2.02, as of this writing).
You should have several BIOS options for fan speed. Optimal, Maximum Cooling and for some tower servers, a Reduced Acoustics mode. Select the appropriate option.
Evaluate the PCIe expansion cards installed. If you have certain cards in place, the system may ramp fan speed up regardless of the BIOS setting.
What disks are you using? if not using HP drives, there's a chance that their temperature is reported incorrectly. This can also be a source of issues.