internal engine sounds are usually rythmic. get louder and quieter in a pattern.

listen with broomhandle. 

a clattering disorganized sound would be external -- AC, power steering, etc
    take the belt off and start the motor
scattered disorganized sound could also be torque converter, loose bolts, flex plate, etc (will go away in gear, come back in park/neutral)

tapping sounds -- lifter noise, bad lifter, broken valve spring, rocker is shot. often changes with rpms (also possibly loose timing chain)
ping noises down low can sound similar, cracked piston, excessive space between rod and piston
to tell the difference, apply load, vary gas at steady speed, a piston noise will change do to the load, lifter will not. if noise goes away, let off the gas, if it gets louder again it's the piston.