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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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Have you checked whether all chainring bolts are correctly tightened? Have you checked whether your crank bolts are correctly tightened? Does the "thunk" always happen at the same point in the pedaling cycle? – arne Aug 26 '13 at 7:29
Is this only when you are pedaling with some force, vs just idling the crank around? Do you sense anything in your feet when this happens? There are two likely scenario: A loose crank arm, which will cause damage very quickly (if it already hasn't occurred), or a loose BB cartridge, which is annoying but mostly harmless. You can also get a similar sensation if the chainring is loose on the spider. Take it to the shop where you bought it and get it checked out. – Daniel R Hicks Aug 26 '13 at 11:41
Ended up being the bottom bracket slowly coming loose. Once I was able to detect some rock in the crank arms I got everything tightened up at the shop and every thing was OK. Arne, Post an answer and I will accept it. – Constablebrew Sep 30 '13 at 18:24

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.

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Mine sometimes needs a kick - tightening the bolt further doesn't appear to help much. – Chris H Sep 25 '13 at 13:04

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