The Ultra 400W unit that's in my secondary PC made a loud squealing noise the first time it was powered on. It was intermittent for about the first week or so, then disappeared all together. It's been over a year and the noise has now returned. I did a Google search and found others with the same problem. The general consensus is that is a capacitor that's going bad. I've purchased a Cooler Master eXtreme 500W to replace it so that I can RMA the Ultra. I've only had a couple PSU's fail on me. So far I've been lucky, every time one has died it went all at once and did no other damage. The only hassle was the downtime required to purchase a new supply. I figure if you've got one that's giving you a sign that something is wrong, best bet is to use the warning as a hint to get a replacement and swap it out.