It's an interesting and common thing to be sure. I work in an "IT" based environment and when HP announced their C7000 blade chassis (the nerds will know) the cooling fans were so well engineered, they actually programmed into the controller a "skip speed" so the fans (modeled after R/C ducted fans) would avoid running at a specific speed and not sound like an F16 taking off. The skip speed was the rpm at which the fan went resonant.
