Clunking noise from pedal, any ideas?

[Turdus philomelos]

Does anyone have any ideas what could be causing the clunk I get from my left pedal area when I start to put pressure on when I go up hill. Its definatly not the noise you get when your lagging.
Cheers

 

[Garz]

Spring, cleat?

 

[youngoldbloke]

Loose?