On the euro coins, the metals ARE the sound source. On keyboards, the possible sound sources are the springs and the plate. The other components (switch shells and PCB) act as coupling materials. In any case, theories don't matter so much as long as the solution works. Anyway, resoldering is a good solution for an isolated switch, and the foam is good if the entire keyboard has a pinging problem.
Ripster, you mentioned earlier that you don't have a pinging switch issue before to try the soldering on. Now that you discovered your "/" numpad key, can you please try resoldering it and provide before/after sound samples ?