I have a customer with a pc that was built by their previous support business. It is emitting a two-tone (sounds like an octave apart if that helps) sound from inside the casing. I've taken off the cover and can't identify where it's coming from. Has anyone come across this before. The noise is intermittent and tends to start when the customer is busy doing something in Windows Mail, or online. It has an Intel Core2 CPU6600 and Nvidia graphics card which is being driven by GEForce 8500Gt driver. The motherboard says it is Abit AW9D. Hard drive is WD4000AAKS-00TMA0. I'm not sure how much more info you might need?