That is never good unless your comp is in a nice and cold room... that means that the psu fan is dead more or less. My moms computer did that. made this awful grinding noise at boot some times while it was on and then you started smelling burning silicon. the reason why it stops is that the fan is probaly dead(stopped) while your comp is on put your hand behind the fan on the psu and see if any air is being moved quickly. if it fells like a small breeze your fan is probally dead. as what ever air compresion you have in your case is pusshing out that hole. If your computer is in a cool enough room you can opererate your pc with the psu fan. look at my server. the fan in it died ages ago and its still running with out problems. short replace psu they are cheap enough. rosewill value serise 400w cost about $25 on new egg. got that for my comp from a 250w to 400w and gave my mom my 250w as a replacement for her dead psu.