Yeah,
I intended that the flat panel from which the exhaust protrudes is the cause. My thoughts come from performing the same excerise as you SPS - Removing the leftover hydrocarbons to prevent corrosion in the rivets along the tail. The film buildup on the panel is the evidence of the pressure change. It is only a line of thought and I am sure frank has checked it out....
I remember now that the Appollo spray rig is attached to belly mounts, so of course it would work for the pods.