Answer:
The answer to your question is: When you touch the vibrating metal, the flesh of your finger absorbs the energy quickly, so the sound dies away quickly, pretty much instantly in fact.
Step-by-step explanation:
When you set a bell ringing, the metal body itself vibrates. It launches those vibrations into the air to create the sound you hear. Because the air is comparatively thin, it loses energy only slowly, so the sound dies away slowly.
I hope this helps and have a wonderful day!