Thud.

[tommyb]

I think I sense a thud. 
A silent throb
Now sounds. 

It slowly, thickly spreads. 
I think I sensed
A thud. 

(I think I spoke without much sense
Of thuds, for soon the thrill subsides.)

I love the line, "It slowly thickly spreads." Maybe it's my synesthesia talking, but that is a perfect description of the sensation I feel upon hearing a soft thud in a quiet place.

The form is very effective. Simultaneously a sense of depth and fleetingness. The parenthesis is a lovely touch.