Does your power supply have ECASO technology? ECASO technology keep fans running for cooling down your internal case and power supply temperature for 3 minutes after shutdown. It's a long shot, but something to consider. Not likely, though, since you're just noticing this now.
I've always taken the approach that troubleshooting power supplies isn't really worth it. The risk/reward just doesn't justify messing with it. Plus, a replacement can be acquired for a fairly inexpensive price assuming you're not someone who has to have name brand/latest and greatest hardware.
Of course, that's assuming that the problem is in your power supply. If you don't have a warrenty, I would suggest acquiring a different power supply (either from retail or from someone who has an extra) and seeing if that fixes your problem. If not, move on from there.
