I get a loud noise coming from my system every now and then. I can't make out where it's coming from. It definitely sounds like something is spinning really fast - like when you put a CD in the CDROM. But I know it's not coming from the CD-ROM or DVD-ROM because there are no discs in them.
Where is the noise coming from??
I think it could either be the fan, Hard Drive or Processor. I'm not so worried about the fan but I can't have my Hard Drive or Processor in bad condition. Which one can it be, most likely?
The processor has no moving parts (other than electrons), so that probably isn't the source of the noise. Fans, however, very often get laden with dust (or worse, if you have ants, roaches, geckos, or smokers around), and are necessary for cooling the processor, so you should be very concerned about the fans.
Hard drives can sometimes become noisy, but since they are pretty well sealed, that usually only happens if they are subjected to pretty violent movement.
Thus, when your system is off, open it up and carefully examine the fans. Most of the time, a can of compressed air is all you'll need to clean things up. If the bearings on the fan are clogged with dust and dirt, you may need to lubricate them, but be careful not to mess up anything with the oil. In fact, if the compressed air doesn't help, it may just be a better idea to have a qualified technician replace the fan(s).
If it is a loud spinning noise, I would guess the hard drive or CD drive. Test the hard drive by copying and pasting a large amount of files on the hard drive. If you can hear the whirring, then it is your hard drive spinning. More likely though, is the CD drive. In order to test it, put in a CD and copy files off of it to see if it makes noise. If niether of these make the noise, then it is probably a fan like Traveller said.
It could possibly be a normal function of the fan. Many fans nowadays are temperature-sensitive and will spin faster if the temperature goes above a certain point.
Therefore: CPU gets hot --> Fan spins faster (making more noise) --> CPU cools off --> Fan slows down (noise stops) --> CPU heats up ...
Other than that, it could be the hard drive, Does the noise often occur when the hard drive is being accessed? If the hard drive is noisy, it should be noisy at bootup as well, does your computer make lots of noise when booting?
As for a solution, for one thing just tighten all the screws, it could be that a vibration is causing two pieces of metal to bang into eachother so frequently that it blends into one noise. If that fails, replace the part that's noisy or just live with it.
Oh, another possiblity I forgot; make sure no wires or cables are touching the fan(s). An intermittent noise could be caused by intermittent contact with the fan.
It would probably be a fan or hard drive. Open up your case and see if there is any dust or other matter in the fans. If so, clean it up. (My computer was louder than my AC unit, and I didn't know why until I opened the case. There was an inch of dust convering the intake fan.) If it is yur hard drive, do what psycosquirrel said. If it isn't your CD Drive, then the noise is probably coming from these two parts.