Wherever there is movement in an action part, there is a felt punching to cushion the stop of that action part when it reaches the extent of that movement. Over time these punchings become hard and cause noise. Particularly noticeable while playing softly, this thump can be very distracting. In some Asian pianos, the return of the jack to its rest position on the release of the key is quite annoying. Replacing the felts is a remedy.
Usually there is a felt punching in a groove when a spring is involved. After 100 years or so it might be time to replace these spring punchings.