Lula, troubleshooting Drive noises can be difficult -- there are so many potential sources. If none of the previous suggestions pan out, try breaking your stroke down.
Where in the stroke does it occur? right after the start? mid stroke? at end? on reversal? with short as well as long strokes? fast/slow stroke? When you push and pause?
Some odd possibilities: cotter ring (retains clevis pin at fin clew) randomly smacking hull at end of stroke (check condition of ring to see if it is deformed -- if so, flip pin to face opposite side); outhauls swinging from side to side on reversal or at beginning of stroke; stroke at limit of play; fins (or sprocket bases) smacking hull at (or near) end of stroke. Take the Drive out (ashore), operate it by hand upside down and watch the chain links pass over the sprockets from end to end. There should be no lifting or binding. How does the pedal play look? Pedal slack is adjustable through removable end cap -- should be non-binding but not sloppy. These can all produce a variety of noises when amplified through the hull.
Do you actually hear this on the port side or does this take place when the left foot moves forward? I presume we're not talking about anything that squeaks (lack of lube).
) at the back of the well.