Notice the piece of silk string lower left, under several strings along the bridge, and the other piece upper right, under the strings on the top back.¹ These have been put here to prevent "extraneous sounds".²

In the case of the former, the extraneous noise is usually a buzzing sound caused by an unevenness in the qin surface under the strings. It may be necessary to use something thicker such as part of a toothpick (for aesthetic reasons you might consider coloring it to the same shade as the bridge), or even having the bridge professionally raised by the addition of a matching piece of appropriate hard wood.³

The latter extraneous noise is often more like a high-pitch ringing sound. Re-stringing the qin or simply tuning it slightly higher or lower may cause this to go away. In some cases it might be better to insert the extra piece or pieces of string close to the feet, or simply to take the end of a string (it must be somewhat long after wrapping it around the foot and tying it) and tuck it underneath. It may be necessary also, or instead, to isolate the strings from each other by threading the piece of string so that it goes between them.
Footnotes (Shorthand references are explained on a separate page)

1. Thread
The silk string I use here is from old broken silk strings. I use this under strings on top of the bridge following a theory that perhaps it will affect the sound less that would a material not already used elsewhere on the qin, such as cotton or soft wood; I have no scientific basis for such a theory. Using it underneath the string is probably a sentimental choice as much as anything else.
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2. Extraneous sounds (雜音 za yin)
Also translated on this site as "disorderly sounds". For more on what causes this see "buzzing sounds". Related terms include "buzzing noise flaw" (㪇病 xian bing), and "beating the surface" (拍面 paimian). Regarding the causes for such zayin, in addition to those mentioned above it should be noted that using the modern nylon metal strings can put divots in lacquer. The only way to repair this is to relacquer that part of the instrument. People who allow this to happen to antique instruments have also been known permanently to alter the antique by re-shaping the nayin on the inside of the sound box. They may also permanently lower the bridge, since nylon-metal stringed instruments play better with a lower bridge, but this latter alteration is largely reversible.
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3. Two solutions for za yin

Closeup of the bridge on the qin above.

To prevent buzzing of the strings the bridge may have to be raised. Here the bridge under strings one and two has been raised by the addition of a piece of wood glued to the top of the bridge. The area under strings three to seven has been slightly raised by inserting a piece of string underneath them.
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