I have some easy suggestions.
1. I assume you have a clipless mount. Take your foot out and pedal. If the noise stops, the problem is in your toe clip or clipless mount. Lubricate and adjust it if possible, and see if that stops it.
2. Take some lube and lubricate the pedal shaft where it rotates just outside the crank arm. That may do it also.
3. Make sure nothing is hitting during your pedal stroke, including the back of your shoe.
If none of that works, you may have to check the bottom bracket next. I am sure one of the better mechanics on the board can refer you to the proper procedure to do so.