When I'm stopped at a light, I sometimes experience a thump or buck from underneath the seat. Very intermittent. Since this ain't the smoothest ride, I don't know if it happens while driving. Any thoughts? 2007 Fit sport

 

Could it be the gasoline sloshing in the tank under the front seats?

If so, it would NOT happen just after a fill-up, only when the tank is 1/2 to 3/4 full.

 

 

maybe misfire?

 

Not enough info to work with... Maybe why nobody has been addressing your thread here...