Actually we like to call it periodic motion. Simple harmonic motion generates a pure sinusoind; pendulum, hanging mass spring system, rotation of a point on a circle/wheel with constant angular velocity. For periodic motion it is even more prevalent; your heart beating, eyes blinking, lungs contracting/expanding during breathing, your voice box causing vibrations of the air creating sound, the light around you which resulted from atomic electron level shifts, water or milk during pouring, the motion of a flag in response to the wind, all musical instruments; strings, chambers, reed, percussion ........... I could go on but you get the idea. Check out Feynman's treatment of resonance in his Lectures on Physics for more detailed subatomic periodic motion. Cheers
If you could sample the "random background noise" on a TV that's tuned to nothing in particular, you could analyse the samples and look for a repeating signal amongst the noise - using some kind of harmonic analysis package and a fast computer, say. You should find heaps.
