Damper felt, like hammer felt, compacts over time and use. Since it is in contact with steel strings, it sometimes acquires a layer of rusty crust that makes a buzzing sound when it comes into contact with a vibrating string. Replacing damper felt is a normal maintenance operation for older pianos.
In grand pianos, dampers are lifted from the strings by means of a wire that travels between the strings and through a damper guide rail. Similar to key bushings which wear through use and become hard over time, damper guide rail bushings also suffer from age and use. When replacing damper felt, rebushing the damper guide rails is also a good idea.