Ian, the first photo is of the aft-most section of the tank. The pickup line is copper and could possibly shake and knock against the wall, but I have to wonder what would cause that.

Is it possible that there is a slight suction-side air leak? A burst of air into the suction side might cause that, at least I think so. The other end of the copper pickup line in the photo is near the freshwater pump and a hose runs from the line to the pump. When Amel made your SM, they used some sort of sealant between the copper and the hose and 2 hose clamps to help avoid a suction side leak. There will be no evidence of a suction side leak and no water leaking out, just air going in. See the second photo.