I have a Dahon Helios P8 in brushed aluminum. Looks like this: http://tinyurl.com/6bxwlc
The bike comes with 9 speed shifting via SRAM 3.0.

On my recent rides I have been hearing a strange thunking sound which goes away when you stand on the pedal and not sit on the saddle. Sounds are not always easy to diagnose but does anyone have a guess what it might be?

I have checked all the screws and everything seems tight and does not have any play when I shake with my hands.