My desktop has been making a humming/buzzing noise, on and off. It gets louder and quieter. Any idea what it could be?
Try to use your ear to determine if it is coming from the external speakers, internal speaker, hard drive or fan.
Nearby cell phones can cause interference with speakers that are not adequately shielded, so if it's coming from external speakers, see if keeping all cell phones at a distance helps.
If you think it might be from the hard drive, you can shut down completely, then unplug power from the hard drive and turn it back on with the hard drive disconnected. If you determine it's from the hard drive, make sure you have all your data backed up and get it replaced.
Fans can start making noise, especially after they collect a lot of dust. If blowing them out does not help, you can replace them, except for the fan inside the power supply, which is not generally opened.