Atomicsoda

You can leave your computer on indefinately if you want, just turn the monitor off the conserve power and be sure to have a Standby Power Supply in case of power surges, brown outs, sags etc... which applies even if you are working on the PC.

The loud buzzing sound however could be a sign of a faulty fan either in the Power Supply or on the processor heat sink.

Take the cover off the PC and identify where the sound is coming from and check that all your fans are actually working.

Heat will over time damage your computer.

Cheers

Angela