A bass-like sound, that sounds like any of the following:
* the sound of a pedestrian being hit by a car
* a funky bass-line
* the repetitive sound of a heartbeat
An apparel and gift company in Cambridge, MA, called "Thoomp!" was named after the onomatopoeic "thoomp" sound.
Thoomp!'s company logo (www.thoomp.com) suggests the primary definition listed here.