Controlling HP ProLiant DL380 G6/G7 fan speed

Solution 1:

You can set a power policy via the iLO configuration. By setting the CPU power modes to lower settings, the fans may also spin down accordingly.

Solution 2:

The issue here is related to PCIe riser population. I worked with zero installed cards to a fully-populated enclosure. At around 3 installed cards, the fan speeds settled at higher levels. According to HP, this is by design. It also depends on the types of cards installed, and their power draw. My worst systems had legacy PCI-X risers installed with 5V cards or were running 4 or 5 HBAs and a PCIe flash device.

Edit: Newer BIOS revisions on the G6 tower models (ML350/370) now include a "reduced acoustics" mode that lowers fan speed and noise output considerably.