Here is the spec of my pc: http://www.newegg.com/Product/Product.aspx?Item=N82E16883147032&cm_re=p6210f-_-83-147-032-_-Product
As far as I know my psu is 300w according to factory default.
Now I remove my new graphic card and it seems that there is no longer loud buzzing. So I am assuming that the problem may be caused by either:
1. PSU undervoltage
2. Graphic card defection
Am I right?