My computer freezes and make a loud buzzing noise then i need to disconnect the power and it starts up fine?

and then 3 days later it freezes again i have had this problem scene i bought it refurbished from a guy on ebay he sent me a new one that i have laying around but i dont want to reinstall everything again because i have done 3 fresh installs of window on this pc i am thinking its hardware but i dont no how to find out what it is it is lenovo thinkcentre m91p with upgraded ram

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