1. Bridge placement. 2. String tension. 3. Action — how firm the strings feel. 4. The width and smoothness of the notch in each bridge. 5. How well the bridge feet seat on the soundboard, and their friction. 6. The tightness of the tuning pins. Buzzes, rattles, slapping, bridges drifting or jumping, woody or metallic tones, double notes and resonant noise generally need three or more rounds of adjustment, each taking three to seven days; an instrument usually takes fifteen to thirty days to settle. This is the most tedious and time-consuming part of the work. An experienced teacher makes it look easy, but it cannot be done without a foundation of experience — which is why it is worth buying through a teacher you know and asking them to adjust anything that comes up.
Why a new instrument keeps drifting. Between the factory and your home it has been through transport and changes of temperature and humidity: new strings are still stretching, the bridge feet are bedding in against the soundboard, and the board itself is adapting to your room. All three happen at once, so the tuning cannot hold. This is not a defect — it is a stage every wooden instrument goes through.
Tune daily for a fortnight, then weekly. String stretch is most obvious in the first two weeks, so check daily; from the third week, once a week; after a month most instruments have settled. Work from the bass strings to the treble, turning each peg only a little and going around two or three times so tension evens out. Do not force a string to pitch in a single turn.
Three things you can do yourself: tune it, check that the bridges have not shifted, and wipe the sweat off the strings. If a bridge has been knocked out of place, move it back to the position marked at the factory — no more than that.
What belongs to a technician: slapping, buzzes, woody tones, double notes and resonant noise. These usually come down to how the bridge feet seat on the board, pin tightness or the state of the soundboard, and they have to be heard and tested in person; a written description is not enough to judge. Improvising here tends to turn a small problem into a large one.
Help the environment and it settles faster. Keeping humidity between 40% and 60% makes the biggest difference: away from radiators and air-conditioning outlets in winter, a dehumidifier in the damp season. Left alone in a room that swings between very dry and very damp, no amount of adjustment will hold.
If you are unsure about the first three months of adjustment, tell us the model and your region and we can arrange for a teacher near you to take a look.