I am using Windows 7 Professional 64-bit with an ATI Radeon HD 4870 video card with latest drivers (Catalyst 12.4). In normal desktop Windows use, the fan of my video card powers down and is so quiet that I can't hear it over the PSU fan. It only rises to an audible level when it is under heavy load, e.g. when playing 3D video games or working with 1080p video.
My system is configured to put the monitor into sleep mode after 20 minutes of inactivity (this is set in Control Panel > Power options > Edit plan settings > Turn off the display). Every time this happens and my monitor goes into standby mode, I can hear the fan revving up to a level suggesting the video card is under medium to heavy load. From checking different fan speeds, I would estimate it runs at about 50-70% when the monitor is in sleep mode, while it is at 20-30% on normal desktop use. As soon as I move the mouse and the display turns on again, the fan slows down to its regular speed.
I have no active screensavers, and I have determined that the fan still revs up when I quit all non-essential software. It does not happen even after long periods of idling if I don't set the display to be turned off.
I have run out of ideas on where to look for this problem. There seem to be no suspicious settings in the Catalyst software or the Windows display settings. How can I trace the cause of this issue, or does anyone who has experienced it too have any clues?