My old slotloading iMac G3 made a noise like that before it got warm, and it started doing that pretty early on. Somewhere along the line (long after it was really retired and relegated to backup computer) it started having trouble turning on. Sleep was no problem, but when completely shut down it required being unplugged for a few minutes before starting up again. This went on until one time, when it had been unplugged for a long time, I plugged it in again. It went bang. Literally. Needless to say, it didn't start again.