Fan is running very fast and loud all the time from touching the power button onwards

[Webskater]

Yesterday I turned my computer off. Today I turned it on. I pushed the power switch and immediately a fan started running very loudly. You can see the fan at the back of the computer. I have tried restarting. I have tried switching the computer off and switching it on again - several times now. As you touch the power switch the fan starts running. So, it cannot be anything to do with overheating because it happens when the computer has been off for hours and it happens the instant you touch the power button. The fan then just stays on - it's really loud and driving me nuts. Any ideas why, suddenly, it has decided that the computer's on/off switch is also an on/off switch for the fan? The only thing difference between yesterday when the fan was not running (usually, the fan hardly ever runs - even when the computer is on for days at a time) and today, was that I hoovered dust off the vents at the side of the computer - just a gentle run over with the hoover brush.

[Webskater · Accepted solution]

Well, I'm going to answer my own question just in case it helps anyone else. I took the side cover off the computer and, with a tiny paint brush and a hoover, dislodged 9 years of accumulated dust. Parts of what I assume is the motherboard had a fine layer of dust (possibly partially short circuiting things?). There is a fan inside over what I assume is a heat sink - big circular thing with spiral 'tines'. This was caked in dust. I gently prodded and hoovered and got it all a lot cleaner. Switched it back on. Magic - fans running very quietly. A couple of minutes after turning on, one of the fans seems to have stopped and the computer is back to the beautiful, near silent thing it has always been. Lesson learned. Clean the dust out every 9 years without fail! Also, and this may be a coincidence, it booted in about 20 seconds. Before the hoovering it was often hanging on start-up - with a second start usually getting things going - but with a wait of 2 to 3 minutes before I could log in.