Recently I had a sudden problem with the fan in my 13" Macbook. It had been running fine all morning, but when I left the room and came back, it was suddenly roaring loudly. Fearing that my fan had died, I shut down the computer and let it sit before rebooting it once more. The problem persisted, and I called apple care.
By this time I had looked at the readings given by iStat Pro and noted that when the fan was making the noise I was hearing, the program reported that the fan was running at 0rpm. The tech support operator had me do a PRAM reset, and once this failed to resolve the problem, he booked an appointment for me the next day at the local Apple store.
I went in and was greeted by a genius who wasn't the world's most personable...person. He assumed I was ignorant and tried to tell me that since he could hear nothing happening at that time, that there was no problem with the computer. I showed him this video I had taken in advance in the event that he might say exactly that.