A B737-300 Flight Crew experienced a roll axis upon lowering flaps for approach and landing. Subsequent inspection revealed a 10 foot by four inch strip of skin material had separated from the top of the right wing aft of the #6 slat.

Narrative

The First Officer was intercepting the Localizer to Runway 27 at SAN. At 210 knots; 4000 AGL and 15 DME; he asked for and I selected flaps 5. We felt what we thought was wake turbulence; aircraft roll which eventually dampened out; and heard what we thought were the flight attendants securing the forward galley; as they had been told to do several minutes before. I later learned the flight attendants also heard and felt something at the same time. Sometime later during the approach; I noticed some yoke displacement to the left; which I have seen before at higher flap settings; but I remember thinking it was quite large. After arriving at the gate and the completion of the Parking Checklist; I remarked to the First Officer that I was going to inspect the airplane for bird strikes; as we had flown past some birds on the right side of the aircraft on takeoff roll in ZZZ. I walked out onto the deck of a catering truck; which had pulled up to the forward service door. I immediately noticed a red discoloration on the top of the right wing; just aft of the number 6 slat. I went down to the ramp and a Ramp Supervisor pulled a belt loader up to the wing so I could take a closer look. I found that a 10 foot by 4 inch strip of wing material was missing just aft of the number 6 slat; exposing honeycomb material. It is my opinion the aircraft's flight characteristics would have been more compromised in the clean; flaps up configuration which we never had to experience; except momentarily. I contacted Dispatch and Maintenance Control who wanted pictures; which I took and forwarded with my cell phone camera. A logbook entry was made. Eventually we swapped planes and continued our scheduled flights. I would like to point out that we don't necessarily inspect the top of the wing; nor is it visible from a walkaround at ground level. In this case; I was looking for something else; and the honeycomb was disturbed so much that you could see it from the ground. I assume that Boeing is going to figure out why this strip came off and come up with an inspection criteria and interval.