Another serious problem is the presence of strange rattles or buzzes. The soundboard, which is the large wooden board you can see from the back of an upright piano or from underneath a grand, has ribs glued on it to strengthen it. Sometimes when the soundboard gets cracks in it the ribs come unglued from it in places. This can allow the soundboard to rattle against the loose rib as it vibrates. This can sound like a speaker distorting when it is played to loudly.
Pianos have a wooden bridge which is attached to the soundboard and has the strings running over it.
The bridges have two pins for each string to hold the strings in place. Because there are so many pins very close together, sometimes the bridges split and allow the pins to become loose. This allows the strings to rattle against the loose pins. Bridges are often made in sections that can come unglued from each other also causing buzzes and rattles. Look at the hammers for deep grooves cause by the strings.