[Sam Dezeeuw]

Using the Puppet Pin tool is certainly one solution. Personally it is not the way I would do it though.

For a more organic feeling movement, I would use the Turbulent Displace plug in to your string layer.
-Raise the Offset Turbulence point to the top of the string
-Raise the Amount to about 50
-Keyframe the “Size” to get your movement.

Hope this helps!