1. Bridge placement.
2. String tension.
3. Action — how firm the strings feel.
4. The width and smoothness of the notch the string rests in.
5. How well the bridge feet match the soundboard, and their friction.
6. Tuning-pin tightness.
Rattles, buzzes, slapping against the soundboard, bridges sliding or jumping, woody or metallic overtones, double notes, resonant noise — these usually need three or more rounds of adjustment, each taking three to seven days. A guzheng generally needs 15 to 30 days to settle. This is the most tedious and time-consuming stage, and the one with the most steps. An experienced teacher makes it look effortless, but it cannot be done without a solid grounding.
So where possible, buy through a teacher you know, and ask them to adjust the instrument if problems appear.