I once had an issue similar to yours not too long ago, although on an MTB with single chainring. Whenever I applied pressure to pedal, I heard this noise which sounded like my chain was rubbing up against something - like a slight grinding noise. With no pressure applied (spinning freely), it was gone. After trying everything (swopping chains with my other bike, tightening up chainring bolts, swopping rear derailleurs, new derailleur pulley wheels, swopping BB's, servicing pedal bearings, tightening crank arms etc...), I eventually found the culprit to be the chainring itself. Although the chainring was noticeably flat when placed on a level surface and no visible bent teeth, I decided to swop it out with a spare chainring of mine....and it solved the issue. The issue turned out to be "chain suck", not because the chain or chainring was dirty, but purely because the chainring bent slightly when pressure was applied to pedal. This caused the chain line to shift off axis to the rear derailleur, resulting in the chain rubbing up against the derailleur cage. Still not sure how the chainring manages to fail this way - I probably bashed it on a downhill run without knowing.