A piece of vacuum line works pretty well at localizing sounds. Some of the mechanics aids have a longer probe which is helpful. A long screwdriver, with the tip held against a suspect part, and the handle to your chin can help localize sounds also. Sticks of wood, scuh as a length of dowel, will do the same thing.
I know if I bought that 'stethoscope', I'd never have to use it again, so maybe it is cheap insurance...
tom