whooshing noise

a few days ago my iMAC screen went black briefly, came back on and then for several minutes there was a loud intermittent whooshing sound. Now when I boot or while I am working there is an occasional brief (few seconds) whooshing sound. This hasn't happened before. Any idea what is going on?

that certainly doesn't seem normal - my guess is you've got an overheating problem and you're hearing the fans kicking in

sounds like it could be a fan but the computer feels cool