My Windows PC makes a loud whirring noise when it is switched on. This loud whirring noise persists for about five minutes, after which it goes away. There don't seem to be any problems other than that, just the noise. It doesn't happen every time I switch on the computer, it's only when the computer's been off for more than roughly half a day that it makes the noise on startup. If I've been running the computer for a while and switch it off and immediately back on again, there is no noise. I've tried dusting the inside with compressed air, but it doesn't seem to help. Any ideas/suggestions would be massively appreciated.
You have a fan that is going out.
Start the computer with the side off .
Carefully listen to see if you can determine which one it is.
You can use a paper towel cardboard tube to isolate out other sounds like a stethiscope.
Or carefull touch each fan ,stopping it. if the noise stops you found the culprit fan.