For a long time now, I've noticed a VERY unusual sound coming from my computer speakers about the same time each day -- between 5 pm and 6 pm.  I guess the best way to describe it is like the roar of a large crowd at a football stadium.  In the last week, I'm hearing a second unusual sound at startup.  It's very hard to describe, but kind of like when you hear a car accelerating just before it changes gears followed by a popping sound.  This first sound is followed within a minute or two by the roar of a crowd sound.

 

I have Norton Internet Security on my computer.  When I run a complete computer scan, it finds nothing.  But, something has to be wrong. 

 

I unplug my DSL connection every day for about 10 hours.  I'm worried that someone has added the second sound at startup to alert them that my computer is back online.

 

I've searched online and I can't find any references to any similar problem.  I'm very concerned that my computer is compromised.  But, I don't know what to do to find the problem or fix it.  I would appreciate any advice.  Thanks