i had the simular problem with my left crank arm last fall. as soon as i would put forward or backward pressure on the left pedal it would make a tinking sound. i thought it was the pedal (actually bought new pair) but the sound was still there. i didn't know what else to do so i pulled off the crank, cleaned it (didn't grease it), then put it back on and tightened the crank bolts to the point where my hands where beat red and hurt like hell from the wrench. the tinking magicaly went away.

good luck!