I have a Dell Optiplex GX280 Tower (Mid??). When starting up initially & then intermittently throughout the day, There is a loud noise coming from the back of the box.(not a groan, not a bearing sound, but sounds as if getting a slow atart) If I lift & drop the back of the case (lightly) it foes away. If I unplug the Processor/case fan, it doesn't stop. If I rap on top of the PSU, it stops. I have taken the fan OUT of the PSU, but still get the noise. What is left in the PSU to make this noise. Was going to replace the PSU, but upgrades don't seem to have the same power connector locations (this box has the solid metal back with prepunched holes for power cord in upper rear left of box. Also, have power switch, but computer has switch on front. How does this work w/ 2 switches?