I don't know what the answer is but I have found the following on another website which you might or might not recognise
Your foot slapping problem is similar to mine, which also resulted in metatarsalgia in my left foot. I finally figured out that I was concentrating too much on running B-H-B (Ball/Heel/Ball) and I had not yet learned to keep my feet beneath my center of gravity, so what I was doing was reaching out in front with my legs, then trying to force my feet to land B-H-B. This is not how feet were meant to be used!
I now realize that if I keep my knees slightly bent, keep my feet beneath my center of gravity, and run "from my belly" that my feet naturally land with a B-H-B movement and I don't even have to think about it! Plus I don't have to slam my feet down to meet the ground - I just wait for the ground to rise up to meet me!
Tks for that Granty - I have a perfect gait on my left foot so not sure if it is "centre of gravity". The shin feels painful when the noise starts and my leg feels like a lump of wood. Am I suffering from shin splints? Any clues?