I have an EMachine & it will buzz & shut down. The buzzing continues until I turn the power off completely for a few seconds. Then I can turn the power back on & it will restart but this eventually happens again.
See if you can locate where its coming from. I suspect a fan myself.
With the system running you can stop a fan breifly with your finger, pen cap, toothpick..etc, if the noise goes away you've found you bad fan. You can check the power supply fan(s) this way too. abreif stoppage will not hurt them. Do not stop the cpu or gpu fan for long thou.