As you said it's a 'static' noise so it could ironically be from any internal device within the computer be it a HDD to a fan motor even RAM can do it. Suggest give your computer to an expert, someone who can clean it without breaking it and go from there. For you to do is to check that the external cabling does not run close to exterior influences (of the electromagnetic kind) and check the jacks / connectors are clean, dry and dust-free.