[Stocker]

If you don't have a mechanic's stethoscope, you can place the handle end of a long screwdriver against your temple, then touch the other end on various parts. Water pump, alternator, PS pump...... be careful you don't touch any moving parts like belts, pulleys, or the fan. Should be able to isolate the noise.

I have also read you can do the same with a few feet of vacuum hose, but I've never tried that.