That sound is typical of a fan starting a slow, painful death - possibly a fast death. It's possible that the PSU's fan is starting to fail. If it's the PSU fan, then I'd start being concerned when the sound stops. Try to get a good look at the fan when you start hearing the whirring sound. Are you sure it's coming from the PSU? Could it be another case fan that's in the same general area? If the PSU is top-mounted, then there is likely a case fan in the same general area as the PSU.
It may sound kooky, but you can use a tube to help isolate where the noise in a case is originating. I use the center of a paper towel roll - can't beat the price! Stick one end up to your ear and direct the other end to suspect areas in the case and you can really pin down what noise is coming from specific areas in your case.
Its doing it now, but yes it is 100% the PSU, and its very tempremental. Sometimes (like yesterday) it went all day with no problems, today its just non stop.
New PSU then??
If still under warraty, then I would try and RMA the PSU or get warranty work done on the system. If not, then you could attempt to replace the fan or replace the PSU. Attempting to replace the PSU fan will void any remaining warranty you have on the PC.
If you want to attempt to replace the fan, then you can attempt to read a part number off of the fan if visible. Other then that, the most reliable way to determine the size of fan will be to crack it open and measure the fan dimensions. If you do this, then be sure to BE SAFE. Completely unplug everything from the PSU and give any capacitors in the PSU time to discharge prior to sticking fingers into the PSU.
just a note some Dell systems have odd power supply connectors its worth checking first before spending the cash. nothing worse than spending money and finding its does not fit in the case or on the motherboard.
Yes that's your PSU...i was just responding to David that your PSU is different from the standard ATX. It looks like that PSU was designed/built to specifically fit into Dell systems. You would probablky have a hard time finding another BTX PSU that will fit into your case.