The expensive mistake with a buzzing guzheng is to start replacing strings, moving bridges and adjusting the action before knowing which kind of buzz you have. The four common ones have almost nothing in common.
First, rule out everything that is not the instrument. A frayed pick edge, a fingernail touching the soundboard, a bracelet, a loose screw on the stand, an uneven floor, a phone vibrating nearby — all of these sound like a fault in the guzheng. Take the picks off and pluck the same string with the pad of your finger. If the noise disappears, the instrument is fine.
Second, find the string and the moment. Pluck each string once and note four things: open string or after pressing; light touch or hard; at the attack or in the decay; and whether it survives moving the bridge. Those four answers usually identify the category.
A dry, sandy rasp. Usually a contact problem: a bridge foot not sitting flat, a string not seated in its notch, a rough or rusted spot on the winding, a hairline crack in the soundboard. Move the bridge half a centimetre and pluck again. If the sound changes, it is contact; if not, look at the string and the board.
A wolf note. One note is unusually strong, or turns hollow and its decay cuts off. This is a strong resonance of the body at that frequency, not damage. Shifting the bridge, adjusting that string’s tension, or fitting a set with different tension can reduce it, but not always remove it. Players routinely avoid the note with a pressed note or a change of position.
String slap. A sharp slap or metallic click, usually because the action is too low and the string hits the bridge top or the nut as it vibrates; a notch worn too deep or a bridge worn thin does the same. Light playing clean and hard playing noisy points to action height — tightening the string further will not fix it.
Internal rattle. A hum inside the body, clearest when you tap the soundboard. Tap across the board with a knuckle and listen for a hollow, loose area; if it comes with a buzz when pressing and a suddenly thinner tone, the cause may be internal bracing or a glue joint. Do not open the instrument yourself — take it to a repairer.
A sensible order: picks and hands, then string by string, then move the bridge, then check foot and notch, then tap the board, then decide on repair. Bridges that slide are a different problem — see bridges that move; an instrument that sounds muffled overall rather than buzzing on one note is covered in why a guzheng sounds dull; routine care is in care and tuning. If you are still unsure, send us a recording and we will listen.