One of the fans in my (admittedly old) tower just started making a noise. I t kinda sounds like a diesel truck idling. I of course opened it up and dusted it out with a can of air as best I could but the sound is still here. There are three fans that could be making the noise but I'm not sure which one. If it is the CPU fan, I'm in the hurt locker and need a new one ASAP. If it is the top exhaust fan or the rear exhaust fan (least probable of the three) its still important but not urgent. Any ideas how I can test each fan?