Box humming - Probably the fan on the power supply.

If you open it up and see any other fans, it could be those too, not really familiar with that model to know if they have other fans.

After a few years the bearings get shot and it starts to make noise. You could try blowing it out, it could be too much gunk around it or something, but it's usually the beginning of the end (though it may hum away for years)

Sometimes just moving it around may be enough to get it to seat right again and let you get some quite from it.

Flickering sounds like something has infected your system as mentioned above. Scan for virus and spyware to be sure.

If that doesn't work, check your video settings to make sure the driver has the right refresh rate for your monitor.