If a string produces a sand-like sound, commonly referred to as "wolf tone," it may be due to the string being caught by a gap in the nut at the top of the piano. Simply using a triangular file to slightly expand the sides of the gap can prevent this issue.
If a string does not sound or emits a raspy sound, twist the nylon wrap around it with your fingers. If it moves easily, it indicates that the nylon is not tightly wrapped, obstructing the string's vibration. In this case, the string is defective and must be replaced.
If you notice a sound of the string bottom hitting the pillar after forcefully plucking a string, commonly referred to as "jumping code," it indicates that the string tension is insufficient, or the effective string length is too short, or the code height is not adequate. First, check if the codes are arranged according to the numbering. If the arrangement is correct, you can move this code to the left and then tighten the string.
If the zither body is uneven, check if the ground where the zither stand is placed is level. Try to level the stand; if the legs are of different lengths, simply saw off the excess part. If a modular stand is being used, you can nail a rubber pad of equal thickness in the gap between the zither body and the modular stand to level the zither.
If a buzzing sound is heard from the zither head when plucking the strings, please check if the lid of the zither head case is tightly closed. Poorly fitting lid might be one of the causes. Also, inspect whether the items inside the zither head case are arranged unevenly or touching the in-case tuning pins. Additionally, do not place too many items inside the zither case, as an uneven or cluttered arrangement can cause resonance of the items inside while playing, resulting in noise.