I have a one month old Citizen foldable bike and I have noticed a soft "thunk" when turning the crank. It sounds the same whatever gear I am in. It also seems to go away for a short while if I stop pedaling. This started just last week when my gears were slipping. I had adjusted the derailleur, but the "thunk" remained. I am pretty sure it is coming from the bottom bracket, but it could also be coming from the main sprocket - it is hard to tell.
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An odd possibility, but it happened to me... does your foldie have a kickstand? If so, any chance that your crank is hitting it while you pedal?
Fix for me, once I figured out the problem, was obvious and simple: put kickstand up, push it in toward frame, tighten it down with hex wrench.