A piece of foam weatherstripping.
Non-permanent, non-damaging, reversible, and durable.
22973080_dcp_91495_pri_larg (resized).jpg
Added over 7 years ago: A small piece of foam is placed on the bell or chime strike location, not the plunger itself. The sound is reduced by over 75%, but still can be heard faintly. There are different foam densities that can be tested to find the optimal choice.