horrid clonking from cranks

[BigonaBianchi]

My bianchi developed a horrid clonk from the crank area yesterday. It only happens when I peddal and it only happens when on the smaller of the two front chainrings.
It is a loud clonk, not just a click.

Any thoughts?

 

[PpPete]

Could be one of a number of things

Easy things to check first
Pedals spin freely?
Any movement of cranks on BB?
Chainring bolts all present and tight?

All OK?
Remove chain and spin cranks to see if it's coming from the BB

Of course although it sounds like it's coming from crank area, it might actually be completely unrelated to that part of the bike.