You may have a hard drive problem developing. But a dying hard drive is more likely to make a loud clicking noise rather than a whirring one. To play safe make an extra image backup of your hard drive, just in case the hard drive is the culprit.
Once your hard drive is protected, start looking for a problem amongst the fans. You need to figure out which fan is causing the problem and see if it is loose or damaged in some way. How you do this depends on whether you have a laptop or a desktop computer.
Turn of your computer and unplug it from the mains power socket. Now open up your PC's case and visually check the fans.
You probably have at least two, one to cool your PC's Processor (CPU) and another fan to cool the entire case.
See if they look damaged or loose and turn the fans blades to make sure they move freely. If everything seems OK leave your case open and plug your PC back into the mains and turn it on. Visually check that the fans are turning freely and are not shaking on their mountings. Check that the fans are not touching any cables or wires and therefore causing a noise.
If you can't find anything, heat may be forcing the fan to work harder than it should. Check to see if there is any excess dust in the fans, or anywhere else, that might be causing the excess heat. Heat is also a likely culprit if you don't hear the noise when the case is open (the extra ventilation stops the noise). Use a can of compressed air (you can buy them at any decent computer store) to blow away any dust around the fans. Of course, if you find a damaged fan it must be replaced with a new one to make sure your PC does not start to overheat.
The same job is much trickier with a laptop, which unlike a desktop, isn't designed for user repairs. With the PC running, pick up the laptop and hold it close to your ear. Try to determine where the sound is coming from.
With the laptop switched off, spray compressed air into the vents underneath your laptop, to remove any potential build up of dust from the fan blades. Use only a moisture-free compressed air canister, as any dampness at all could damage your laptop.
Although this technique does not remove all the dust from the laptop it can dislodge it and disperse it - and this can often help!
Don't open a laptop to fix a fan or to clean the insides unless you're very confident about such things. If you decide to give it a try, search the web for instructions (or find a YouTube video) specific to your particular laptop make and model number. If you can't find such help, or if you're not that confident, take your laptop to a computer technician who is used to dealing with laptop repairs.
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