vibrating noise

My wifes computer (HP 511n ) started making a noise that sounds like a metalic vibrating sound a few days ago. I unplugged each drive(noise continuede) and the heat sink fan as well as checked the power packs fan . All are working and the vibration continues.
Is it possable that the heat sink is vibrating on the motherboard or that the CPU is vibrating against the heat sink ? What is the best repair approach ?
The computer is running fine but the noise is very anoying and my wife teaches on-line so she spends alot of time near the computer.
Thanks ; KennJ

It could be resonance, so check that the screws mounting the drives, motherboard, power supply, etc are all tight. If it still continues after that, a good trick is to hold the handle of a screwdiver next to your ear and place the tip on the suspected source - you'll soon hear the component that is making the noise.