Its popular among the fairer FReepers, especially those who frequent live threads.
I believe it represents the sound made by Freepettes when they hit the floor after swooning over various males of the species.
A typical context would involve, say, Major Garrett it tight jeans and a baseball cap.
Crash cart STAT!
*THUD*
Well I wasn't just checking out his jeans, I should get some points for noting it was a Washington Nationals ball cap.