How much louder is "unnecessary"? I only ask because, as you noted, videos (especially if streamed with something like Flash) online tend to generate more heat than local copies.
Assuming it's considerably louder than normal, one of two things may be wrong: either the fans are getting clogged or the fan is malfunctioning. It is easy to determine which of the two it might be just by the sound itself. If it's louder but still sounds normal (something like a song playing but just louder), it could be experiencing issues with dust and other build up. If it's louder but sounds noticeably different (something like a song but with distortion or a broken cable connection), it could be the fans themselves.