Bottom bracket problem

Hi. I am getting a definite 'clunk' when applying pressure on the left hand pedal which I can feel through the frame. I have checked the pedals and they seem to be ok. There doesn't seem to be any play in the bottom bracket either so I am puzzled as to what it could be. If anyone has experience of this and can shed light I would be grateful!
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lose arm?
Parktools :?:SheldonBrown
It could be BB but doesn't really sound like it.