Have you tried using a stethoscope to pinpoint the location? If you can't get hold of one a length of rubber tubing. If the pitch does not alter with engine rpm but is only present when the engine is running then it must be related to something that remains reasonably constant like fuel pressure, oil pressure and vac system. A p-clip somewhere that has lost it's rubber lining, a pressure relief valve that is not seating, like the fuel pressure regulator and damper.