There isn't any fan control settings in the BIOS as I remember. The D1xx range are entry level. See HP DL 165 G6 very noisy - fan speed - HP Enterprise Business Community.I saw the review and expected it to be louder than it is. What are your fan control settings in BIOS? You aren't running MAX right?
I have read that its due to a faulty sensor on the front backplane. Not sure that's just a red herring.