I have a Dahon Helios P8 in brushed aluminum. Looks like this: https://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.

is the pedal hitting anything?, also check the gears and the seatpost, if it goes away when you stand up and pedal it must be something to do with the seatpost.

also try pedaling standing up if it still does it then you can safely say it is not the seatpost

Check your seatube for play. Also check the frame in that area for cracks.

Thanks for the suggestion guys. Will check when I get home tonite.

no problem

Put a piece of paper or vinyl tape in the face of the rebar.

jur got it

tyvek ( export shipping envelop ) works best
thor