Buzzing Noise while Watching Videos in Acer laptop

Hi, this is perplexing. Whenever I watch videos, my Acer laptop would produce a buzz-like noise and after a while, would hang. I already added a video card (ATI 5450) but it would still do the same thing. I already did my updates on all my drivers. By the way, I am running at Win7 64bit. What should I do? I racked my brain and the internet but to no avail. Your ideas will be highly appreciated.
