Loud whirring noise when mains switched on but PC still off

When I switch the mains power on for my Dell 92000 system, the fans appear to start and they seem REALLY loud. That's it. There's nothing on the monitor, and the monitor light is in fact still orange (as it would be if the monitor was on but the PC switched off).

Sometimes, the fans seem to be getting faster and faster, and I switch the thing back off when it starts to sound like a plane starting up.