When in an engine, it's typically repeated rhythmically. On carbureted cars, check out the fuel pump before you start opening up the motor. When emanating from the nether regions of the front end during a turn, this sound may be traced to an outboard CV joint. A heavy bumping sound, softer than a clang, usually indicates you should look at suspension bushings, including shock or strut mounts. If it's not due to a colony of bats under the hood, maybe a belt's coming apart.
Fan interference is another possibility.
Regardless, this is a visual inspection sort of thing. A horrible, torturous sound, like a bad dentist would make while working with obsolete equipment, means something's going awry - and fast. If it occurs when the brakes are applied, either the linings are gone or you've got one of those unpleasantly-aggressive friction material formulas that tend to eat rotors. Something's dry, probably a suspension component. If it's metal, it's going to break really soon.