Thanks givemewings, hopefully that's all it was. The sound didn't strike me as being one that plastic would ordinarily make, but of course it's difficult to say that with any degree of certainty.
I should stress again, though, that the sound was primarily emanating from the upper left and left side of the fuselage - neither me nor my wife noted the sound from the exit doors we were sitting closest to, which were on the right side of the plane.