This is a vibrating string simulation that is accomplished with a simple oscillating expression applied to a slider control, which is then linked to the Tangent controls of the Bezier Warp filter (AE Pro version only).
You start by creating a long, narrow solid. You apply a slider control to the solid and apply this expression to it:
amp = 15; //amplitude (pixels) freq = 10; //frequency (cycles per second) amp*Math.sin(freq*time*Math.PI*2)
Set "amp" to the desired amplitude and "freq" to the desired vibration frequency.
Then apply the Bezier Warp filter and apply this expression to the Top Left Tangent, Top Right Tangent, Bottom Right Tangent, and Bottom Left Tangent:
value + [0,effect("Slider Control").param("Slider")]
For this example, I duplicated the solid twice and changed the frequency value for the copies.